Good question. I was doing some research on respringing for my oversize.
I'm 6'2, 230ish
The recommended spring rates for me were:
Front: .480 kg
Rear: 5.40 kg
The YZ 400 stock are .460kg (close to mine)front. And 5.40 rear (exact)
The 125 stock Spring rates are:
.421 Front
4.8 Rear
So the 400's rates are close on the front, and exact on the rear for my recommended rates.
I happened to pick up a 99 400F for a part out, so it became a donor.
Also the 400 forks have a better routing for the brake line before it connects to the caliper, not as exposed to rocks, etc.
I was going to send them to Factory Connection, then I thought about when I go to sell it.. I couldnt justify almost $1000 to get it set up perfect for me, just to sell it to someone that would need them rebuilt to thier size anyway. So the donor is the route I went.