Boost leak it might be that its overfuelling then but you should be able to see it smoking then when power is reduced

Ignotion might be retarding you because of something?

Bad turbo can be checked. Hose of and check play in the shaft. No smoke? No damage on the turbo wheels? No running from the wheels onto the house?

It might be your turbo performing less. Going onto a different place of your compressormap causing more heat build instead of compression. Still pressure is the same but it has more heat, so in the end less engine power