I think its more about packaging and price than anything else. Its a pain to to do a front mount in 4th gens because you'll either have to lose AC, move or get a new radiator, use different fans, lose access to plugs, etc. And for a quality kit be prepared to shell out at least 5 grand on the low end. STS kits are a compromise but a pretty good one, if you know how to get the most out of them. For one, no LTs! Stock manifolds or coated and wrapped shorties are the way to go along with coating and wrapping the pipe all the way back to the turbo, gotta keep that heat in. Also, you'll have to run a lower A/R that a front mount so you keep lag to a minimum. However, this may all be moot depending on how much room can be gotten out of the 5th Gen's engine compartment. Twins anyone?
