Volvo D12 short of power gears 9-10

We have 2 Volvo D12 pulling same size grain trailers. Night and day differance between the 2. The power shortfall is most noticable in gears 9-10. In one mile, there is a 7-12 mph differance, depends on headwind. Both trucks have 425 hp. Had Volvo dealer get hp up to same level. We replaced a EGR valve based on code received. Dealer pressure tested system and identified leaks in the air intake system coming from the turbo. They replaced leaky gaskets, installed a new air to air unit (due to leaks detected) and we replaced turbo, again due to a leak on the fresh air compressor side. After all of this, still terrible performance, most notably in gears 9-10.

Ideas??

 

Thanks. We have less than 3000 miles on fuel filters, but do plan to change them again. Local Volvo dealer did computer check which showed normal fuel pressure levels, but not sure if the pressure reading would detect a faulty fuel overflow valve on the return line. I assume it would??