No, I hadn't tried that. The header and exhaust are held together with a silicone coupling, and they're already smushed together about as close as I can get them; the coupling is already pushed against the bend in the header and the widening chamber of the exhaust pipe, so if I were going to make any changes I'd need a shorter coupling to start with. Maybe someday, but I'm content with 50mph from this car.
It's coincidental that this thread should pop back up right now, because yesterday afternoon I made some of the same modifications to a Losi 3.4 engine (mostly just increasing the compression until I stopped seeing speed gains) that I have installed in a different RS4, and speed tested it:
Based on my calculations (65mph speed, 66mm tires, 3.96:1 drive ratio), the Losi 3.4 was turning ~33,333rpm, which is very close to its rated maximum of 34,000rpm. I doubt I'll get more speed out of that car, maybe 1-2mph at the most. I might shim the sleeve up by 0.1mm to see if the extra port timing makes any difference, but I doubt I'll do anything to the crankshaft on this engine; this is already my fastest vehicle by a 30% margin.