By my recollection, PD tried some new stuff with GT:C (namely the zonda sounds!). It sounded tons better than GT3, but was still missing a bit of that meaty resonance you get from a big 8.
Bout the gears, a straight cut gearbox isn't used to change gears without the clutch. You can do that with a regular gearbox easily enough by rev matching (all that heel toe shabang isn't performed for nothing). I learned to shift without the clutch in a suzuki swift!
A straight cut gearbox is used to recover the appauling power loss through the transmission. A regular tranny will lose about 15%, but a straigh cut will only lose about 5-10%. Only catch is it makes a LOT of noise doing so (that 5% is noise

). It drowns out engine sounds in anything up to a v8 supercar (watch that on tv, they really have noisey gearboxes that masks a 650hp 5L v8!).
Back to sounds. You could genericise the sounds but modify them with a snyth. You could use a simple flag system to say which effect and how much, and you could tweak those flags with the modifiable parts, following the same procedure for every car (almost!).
For instance, you would definitely have a few modifiers to make the sound more interesting according to rev range. You have idle, coming onto cam (boomy exhaust note kicks in!), then intake whoosh, maybe modified further up by a "crapness" (technical term) to alert you to shift. In reality you can hear when to shift in most cars (rotaries excepted) purely by the engine note. You also get the boomy exhaust note from lifting off. This is caused by shutting off the inlet, which gives you engine braking (See: negative torque, compression braking, etc). It's this that brings a sample alive. Samples by themselves are crap.
Those are the simple generic adjustments, but you could change the levels applicable to each per car without costing much memory or space.
Then the mods. A stainless straight through exhaust has a hell of a lot more boom! Most race exhausts provide this change with very little exception. Simple modifier flag. Inlet changes to high flow will also be evident in the upper end of the rev range (volume increase).
Changing a subaru from a standard twin tip exhaust to a high flow single tip gives you that proper "dug dug dug" boxer sound. This applies to all boxer engines (vw beetle, porsches, etc)! Porsches have variable inlet lengths, vvt and l, and thats why they make that great raspy rattle. Again, simple modification of a generic "boxer 6" sound.
I think you could get away with as little as 20 base sounds, and with say, 10 flags per car you could have the sounds done. Then throw in some "parts" alterations which could generically effect different engines (saves having to do alteration flags for each car), like boxersound=1, resonance_exhaust=1, vol=8, overrun_vol=6, which wouldn't cause a 4age to sound like a subaru, but would make the exhaust louder and boomier on the overrun.
Just throwing ideas around. Not that PD would ever read this, or that anyone here actually would care.
Now, try to shoot me down...