[AT&T, Rogers HTC One X, Telstra One XL] grayfoxmg1:
Figured I would try my hand at answering this.

Stock kernel is the MOST stable kernel available for use with this ROM. If you have flashed stock kernel(without errors) than most issues you encounter are from the ROM itself and not kernel related. That said almost every CM9 based ROM for our phones will take the stock kernel without issue. If you want to use CM10 you'll need a different one. But what if you wanted better battery life? What if you wanted more hardcore performance? What if you wanted the ease of use provided by Sweep2Wake?

That is where Beastmode of ElementalX come in. Both allow the user to adjust their clock speeds to their desire. Thus allowing you to have better battery life, and more hardcore performance. Both Kernels also include more governor options than available on stock. If you wanted to use Sweep2Wake, than you would also need to flash one of these kernels. ElementalX also allows for OTG support, which the stock kernel does not. Team Blackout (specifically Zarboz) is working on getting non-powered OTG on beastmode kernel. I'm not sure about ElementalX, but I know that full aosp/CM10 support is in the works for beastmode. Those are just the things I can think of off the top of my head.

As for radios, yeah it's based on your specific geographical location, the service in the area, and what you want to get from your device. For example. The 2.20 radio is the best for my phone if I want to use LTE, however it gets some of the worse signal in my area. 2.41 will give me signal EVERYWHERE in my town, but I'm unable to use LTE like I would want. However, because I don't use my data a lot, and am normally around a wi-fi connection, I've decided that 2.41 is the radio for me, because I would rather have to ability to call and text everywhere, than get good data speeds. If you want to see what radio is best for you, you could always check the google maps/layers thing set-up in the radio thread, or download a few of them(they're not that big) and flash them for a while. Go trough a day with each radio and stack it's performance against the one that came on your phone. Is your signal better? Do you have fewer dropped calls? Is LTE faster? I can't answer these things for you, because every area, every tower, every phone, and every users needs are different.

On flashing radios on hboot1.14(hboot from factory 2.20) I don't know where this rumor started, but as far as I know, you MUST downgrade your hboot if you want to flash; kernels through recovery, splash images, boot animations, and more importantly RADIOS. While I am sure there is some ridiculous work around to allow you to flash it from your computer, it is just easier to downgrade and flash. If that doesn't sell you than think about this. None of these radios were designed to be hboot 1.14 friendly. So even if you somehow get one to stick, if you encounter any errors down the road, we(As the support community) won't know if it was caused by you flashing the radio on hboot1.14 or something else. It creates more work for us, decreases that stability of your current build, and increases the time it would take for any issue you have to be resolved.

I could put it another way; with the benefits of having an older hboot, the fact that an ubuntu live cd is free and easy to burn, the ease in which the JET kit is ran, and all of the great support over in that thread (Yarrimmapirate, absoulutleygrim, soulshadow, and more) why wouldn't you downgrade.