> For example if one is up to speed in LSF, can one still make sense of classical latin texts and poetry etc?

I think that's fairly unlikely, because LSF uses modern Romance techniques of word order and prepositions to substitute for Latin inflections. In Latin the word order is relatively free, especially in some famous classical authors.

Check out this passage from the Aeneid:

his ego nigrantem commixta grandine nimbum,
dum trepidant alae, saltusque indagine cingunt,
desuper infundam, et tonitru caelum omne ciebo.
diffugient comites et nocte tegentur opaca:
speluncam Dido dux et Troianus eandem
devenient; adero, et, tua si mihi certa voluntas
conubio iungam stabili propriamque dicabo,
hic hymenaeus erit." - Non adversata petenti
adnuit, atque dolis risit Cytherea repertis.

If I try to remove all of the meaning that's given only by inflections (leaving singular/plural distinctions), this would be something like

these I darkening mixed hail cloud
while shake forces and leaps circle close
from above wetten and thunder sky all shake.
scatter comrades and night hold dark
cave Dido leader and Trojan same
come; be present and your if me sure will
marriage join firm and own call
this marital is." - not opposed requestor
agree and trick laugh Venus found

Looks like the inflections were doing a lot of work here. :-)

OK, how about Galileo's Siderius Nuncius?

Magna equidem in hac exigua tractatione singulis de natura speculantibus inspicienda contemplandaque propono. Magna, inquam, tum ob rei ipsius praestantiam, tum ob inauditam per aevum novitatem, tum etiam propter Organum, cuius beneficio eadem sensui nostro obviam sese fecerunt.

Big truly in this small treatise all about nature speculating to be inspected and to be contemplated propose. Big, say, then because thing itself excellence then because unheard through era novelty, then also because Instrument whose help same sense our apparent themselves make.

OK, that's a little more intelligible -- by Galileo's time Latin prose that's not meant to be fancy or showy is more similar to modern languages, and Galileo presumably was thinking like a modern Romance language speaker, using Latin as a foreign language.

Going back to Cicero:

Magna dis immortalibus habenda est atque huic ipsi Iovi Statori, antiquissimo custodi huius urbis, gratia, quod hanc tam taetram, tam horribilem tamque infestam rei publicae pestem totiens iam effugimus.

Big gods immortal to be given is and this himself Jupiter Stator, most ancient guardian this city, thanks, because this so foul, so horrible, and so dangerous republic danger so often now escape.

I guess you can get the idea here too, but the tenses would be helpful, and maybe being able to see where the "of" and "to" go...

Vulgate?

in principio creavit Deus caelum et terram. terra autem erat inanis et vacua et tenebrae super faciem abyssi et spiritus Dei ferebatur super aquas. dixitque Deus fiat lux et facta est lux. et vidit Deus lucem quod esset bona et divisit lucem ac tenebras. appellavitque lucem diem et tenebras noctem factumque est vespere et mane dies unus. dixit quoque Deus fiat firmamentum in medio aquarum et dividat aquas ab aquis. et fecit Deus firmamentum divisitque aquas quae erant sub firmamento ab his quae erant super firmamentum et factum est ita. vocavitque Deus firmamentum caelum et factum est vespere et mane dies secundus.

in beginning create God sky and land. land however is idle and empty and darknesses over face abyss and spirit God carry over waters. and says God make light and made is light. and see God light because is good and divide light and darknesses. and call light day and darknesses night and made is evening and morning day one. say also God make firmament in middle waters and divide waters from waters. and make God firmament and divide waters which are below firmament from these which are over firmament and made is thus. and call God firmament sky and made is evening and morning day second.

(That one might be deceptively easy because this text is so familiar...)

Cicero's De Finibus, the origin of the text that got corrupted into "Lorem Ipsum"?

Sed ut perspiciatis, unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam eaque ipsa, quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt, explicabo. Nemo enim ipsam voluptatem, quia voluptas sit, aspernatur aut odit aut fugit, sed quia consequuntur magni dolores eos, qui ratione voluptatem sequi nesciunt, neque porro quisquam est, qui do lorem ipsum, quia dolor sit amet consectetur adipisci velit, sed quia non numquam eius modi tempora incidunt, ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Quis autem vel eum iure reprehenderit, qui in ea voluptate velit esse, quam nihil molestiae consequatur, vel illum, qui dolorem eum fugiat, quo voluptas nulla pariatur?

But so see, from where all that born error is pleasure accusing and pain praising, all thing open and those themselves, which from that finder truth and nearly architect happy life said are, explain. Nobody indeed itself pleasure, because pleasure is, scorn or hate or flee, but because follow big pains those, who reason pleasure follow don't know, nor indeed anyone is, who pain itself, because pain is love pursue drawn to want, but because not never his kind times occur, so work and pain big other seeks pleasure. So indeed to smallest come, who of us exercises any body undertake laborious, except so something from her use obtain? Who however or him right blame, which in that pleasure want be, which nothing bother follow, or him, which pain that flee, which pleasure no give birth?

Anyway, I don't think full-scale inflected Latin texts will be any easier or more comprehensible than those above if you learn Latin entirely without inflections. :-)