I wouldn't imagine the improvement would be noticeable in times unless you're running a LOT of power...but the increase in the strength of the chassis is worth it in itself. Cornering is more fun and going up and down curbs makes for a lot less squeaking with the subframes installed. Translation: do it.
------------------
'91 LX Hatch, 17 ROH ZR6's, lowered, 3.73's, offroad H pipe, SN Cobra cat-back, ASP pullies, 14° timing, C&L 76mm MAF, 170A 3G, B&M Ripper on a T5
Best ET pass: 14.593 @ 94.62
Best MPH pass: 14.728 @ 95.72 (75°, 75% hum., 2200')
1991 LX Hatch 5.0L