"I...'d like to get more variety out of the instrument, including some Strat-like sounds... I'd like to get more Fendery tones. Is a Q-filter the answer, or should I be looking at an L-290 or even a 280?"

For the "Stratiest" and most Fendery tones, I'd go with the 280. For a bit warmer tone, but still in the Fender direction, go with the 290.

I'm sure that, in general, the Q-filter is a nice way to sweeten up most any pickup and add a little Strat-like chime and sparkle. (i.e. tonal effect of removing turns et al.). I'm not sure if that would be enough of what you want in the 250, or whether it would simply be best (and easiest) to simply go for the 280. Either way, given the ridiculously high quality to price ratio of Bill's stuff, you can't go wrong.