997 Turbo spots the 458 70hp, so even IF they weight the same the 458 would still has the advantage, by a mile.

Think the 997 Turbo spots the 458 by about 100kg off the top of my head, so it weights 100kg more AND has 70hp less HP in addition to the awd 'handicap' but it's not that much slower, wonder why?

Also, have you check on the drag coefficient numbers of the cars? At speed they plays a BIG role, more so than weight.