3500 is way too high, all diesel power curves (mapped or otherwise) drop like a stone after 4000 rpm, diesel burns slower than petrol and higher revs don't give the charge enough time to complete combustion. Hence the masses of smoke out of the back of old-fashioned diesels at high revs, and the reason some remapped cars smoke a lot.
I'm guessing you have an AMF engine, which does indeed have a wastegate turbo and not a VNT device. The wastegate may be starting to act at around 3500 rpm, but the turbo boost should be felt from much lower down. Do you know if it is an AMF engine or some other code?
This is not normal behaviour for a diesel engine, and I'd have to suspect the remap first. What gain is claimed for the remap in power terms?