Performance Car mag pitted a std Sapphire Cosworth 2wd vs a 2.5 Merc 190 around Castle Combe in about 1989. I think they got Ian Taylor ( Thruxton Race school ?) to do the laps and they were very similar times. His general opinion was that the Merc made up in the corners some of the Ford's advantage on the straights ( was before they put the 2 chicanes in at Combe ).