Here're the actual measurements from 2 recent eBay auctions. I believe the seller is AK member who totally restored both receivers and then bench-tested them. Well documented, both channels driven simultaneously. Here are the links and actual results.
Pioneer SX-1250
http://www.ebay.com/itm/Pioneer-SX-...pt=Vintage_Electronics_R2&hash=item257e8ccb08
Pioneer SX-1280
http://www.ebay.com/itm/Pioneer-SX-...92?pt=US_Stereo_Receivers&hash=item257f5bb36c
Pioneer SX-1250 (officially rated 160 at 8-ohm): 8-ohm load actual rating 185 and 193. 4-ohm load actual rating 269 and 284.
Pioneer SX-1280 (officially rated 185 at 8-ohm): 8-ohm load actual rating 231 and 233. 4-ohm load actual rating 270 and 276.
As we can see, SX-1250 average is about 189wpc, while SX-1280 is about 232wpc, so 43wpc more powerful. In 4-ohm they are about the same.