I've had the Sportlines on mine now for about 6 months.
They are awesome!!! Yes there is a small drop, I'd say about -30mm. Basically it stops the car looking like a 4x4. They are progressive springs so the ride quality in general is not harsh. Start to throw it around a bit more and the (progression) spring handles excellently. Very little body roll. IMO one of the best mods for the money anyone can do.
Compared to the Prolines? Don't know, haven't ever experienced them.
On the box Sport's say -50mm drop (compared to what I don't know). But its not that low, not harsh, excellent feel.
Go for it.....Sportlines all the way......