Look at line 453 of _/catalog/controller/product/search.php_ on v1.5.4 , the code you suggested wouldn't change the behaviour I'm pretty sure as both _$this->data['filter_name']_ and _$filter_name_ will be defined to the same thing.

For 1.5.6 however you have a good point - it does mean the correct value won't be passed which I hadn't noticed before. Though I don't think this has an impact as for 1.5.6 the search keyword is tracked automatically by piwik using the GET argument so the call made to _trackSiteSearch_ is ignored for OC 1.5.6.

This is probably slightly better coding practice to do the changes you suggest, though I don't think it will affect behaviour at the moment. I'll put it as a low priority thing to test out and change for the next release. Cheers :)