I've got an Avanzato as well, there are loads of modifications for it, but you have to be prepared to ship the parts from Japan. The extent of modification all depends on how deep your pockets are.

In terms of the engine, decent gains can be made from changing the air filter and upgrading to a stainless exhaust as the stock ones are very restrictive. There's a few Japanese air filters like HKS available and a drop-in K&N one. Exhausts are a little more difficult as the R4 is rare in Japan. Nearly all Japanese ones are made for the FWD R model which lacks a bend in the pipe etc. Most people seem to get stainless ones custom made.

The next stage would be to alter the level of turbo boost but you need to uprate the intercooler first, the stock intercooler isn't very good and if you alter the boost without doing so it will overheat. There are a few options, if you want to keep the top-mount there's a very fancy Japanese one for about £400. Alternatively a front mount Copen intercooler is said to do the job well too. The boost can be altered through various methods, a good one being a HKS Power Assist kit. Alternatively you can replace the whole turbo, RHF3/4/5 etc.

And if you're not broke by that point there's all sorts extra in Japan, pistons built to F2 racing specifications, stroke up to 713cc etc. You can get a bit over 200bhp out of the JB-JL engine which would give a pretty scary power to weight ratio!

Good luck! And be sure to change the gearbox oil often.

Finally this page might be useful to you, Email me if you need to: http://members.aol.com/shutokousilvia/avanzato/