It doesn't look to me like there's a change in the payouts for PC part referrals. In both the old and new structure, those are a flat 2.5%.
Volume pricing is gone.
PC parts (along with electronics and a dozen or so other categories) were already ineligible for volume pricing under the old program. Volume pricing applied only to items that did not fall into one of the flat-rate categories.
don't put all your eggs in one basket
I hope they have other retailers' affiliate links too, like Newegg.
They do. You can see it via the codes embedded in the links.