Trail is self-centering action of front-end. It's like caster front wheels on shopping-cart, it'll automatically aim wheel in direction of travel. The more trail you have, the stronger the self-centering torque on steering.
By lowering front-end, you make fork-rake steeper and reduce trail. Stock 250 has about 84mm trail, which I think is very short and makes the bike responsive as is. I don't think making it even more responsive has measureable benefits. It won't get me faster lap-times at track, but makes causes stability issues as Dave pointed out.
My CBR600RR has 95mm trail in comparison and doesn't feel as responsive at low-speeds. However, at 100mph, it's much more stable than Ninjette.