I found some inside photos from one of my friends.
Thanks for the look inside.
Mostly it looks good, but I don't like the messy wires in the back.
Compared to Dave's BKP 8601, I'd say this comes from the same design, but has been modified for the higher load capability.
Some differences I see straight away are in the mains wiring and the transformer. On Dave's BKP the mains comes in the back and travels across wires to the front; the transformer is shielded. On this one, the mains travels through the PCB and the transformer is not shielded.
The 8513+ seems to be ~99mm deeper than the 150/300W versions, see 8500+ series datasheet: http://www.itechate.com/Upload/File/20150806091844.pdf
It also looks like it has more transistors on the heatsinks. That would explain the increased power handling capability.
Thanks, that would indeed account for the higher dissipation capabilities.