noel-pilot, erm ehhm well yes a 172 is quicker than an original DB7. The original DB7 running the in line 6 cylinder engine developed about the same power as a the jag xjr (320, maybe nearer to 350) for a weight of 1700-1800Kg. Simple maths produce 177-194 bhp/ton. given the extra weight and crap jag xjs chassis, that car would have NO chance on track against a 172/cup.
now, a newer generation DB7 with 400+ bhp would be a better match, but I wouldnt bet the family silver on a twisty track between even a vantage and a well driven cup.
Ive driven high powered heavy cars for a long time, and when it came to having fun on track, I went for the cup. If I didnt need occasional back seats, would have gone for a caterham R300/400/500 that would wipe the floor with any aston/bmw etc ..