In reply to Stu Gerrard:
Yetis were originally designed to go over Scarpa boots, so never a perfect fit on Sportiva, but they're so old and boots/soles have changed so it's a bit irrelevant now. I have used Yetis on older Sportivas, it works.
Regular Yetis don't really add any warmth, maybe a bit in the leg, and stop ingress of snow. But you lose heat through the sole, into the snow via metal crampons. Fully insulated overboots are relatively fragile and not really for 6000m (unless Denali/Logan). Plus they're a pain in the arse to get on and off at camps, as are Yetis if they slip off.
So I'd look for a 2nd-hand pair of double boots, nothing fancy if you're just going the regular routes. Leather Sportivas would be fine for the approach and lower peaks, but if you're camping on snow at a mid-camp and getting up early to summit, or on Huascaran, then your feet would probably get cold. There's always exceptions, but this is a general guideline.