i have ran both my previous ibiza 8v and now my current 16v with no spare wheel (space savers are not very pleasant, I use a can of tyre weld) and no back seats/plastics. does feel a fair bit nippier, especially on corners and braking! more so on the 16v cos it doesn't have as much torque as my 8v did. although the 8v wasn't too loud, for some reason my valver has a lot more road noise and the exhaust is a lot raspier/louder too.
8v with more torque than a valver..............we won't go into that!!
with the 8v a good mod is to fit a camshaft from a mk2 golf gti (pb engine with digifant) for an easy 10bhp (approx) due to it having a slightly higher lift and longer duration. a power rohr (3" alloy/steel intake pipe) to replace the plastic one helps a bit too