Raymond, as I noted in the NE Forum, there's other things to consider, things that the core numbers won't tell you.
Foremost, remember your target: the E36 BMW. The E36 chassis has probably the absolutely ultimate in development, testing, and aftermarket racing support of any production-based car. BMW and privateers have been developing and testing this car for a decade, and it shows in its success.
The same cannot be said about the A4. First, it's almost impossible to find *any* aftermarket ECUs for the non-turbo V6, let alone racing-based ones. Second, it's almost impossible to find a good racing header for this car. The BMW, on the other hand, has numerous off-the-shelf options for exhausts (a VERY under-rated power gainer for race cars) and I hear that teams are even developing Motronic electronic engine control systems that fit inside the stock ECU box and use the unmodified wiring harness. Just imagine the development costs on those items. The bottom line here is that the BMWs are gaining FAR more than 15% at the crankshaft, but without a lot of ($$$) development the A4 is unlikely to attain +15%.
Moving along, although similar in stock horsepower to the 325 and a lower weight, the Audi would be at a significant disadvantage with front-wheel-drive. Even if given a 200-pound weight break versus the BMW I think one would be hard-pressed to be able to keep the A4 from burning off the front tires in chase (kkpe in mind it will likely be classed at least the 2850 pounds of the BMW). While I'm a die-hard FWD fan I realize the disadvantage in racing, as does most other major sanctioning bodies. When you toss FWD v. RWD into the mix, you've got to look way past power-to-weight as an equalizer.
This is not to say I don't find this an intriguing project; in fact I've got access to a '98 A4 with an owner itching to build one. Mid- to late-90's V6 A4s are coming on the market real cheap, and I think the A4's suspension design is excellent, possibly on par with the BMW's. The brakes aren't quite as good as the BMWs but with good pads and proper ducting could be made to work very well. It would be a fun project, though you have to keep a reality check in mind. If we could get a 150-200 pound weight break over the 325 then I'd be game for building a FWD A4; however, at comparable weights I just can't see spending the money.
Now, since AWD is not explicitly disallowed in IT, if you can get it classified at no more than the weight of the BMW 325 then an AWD A4 would be a horse of a different color, and definitely worth a look...
