Here's a list of the Standard V8s that I've got with weight / pp according to myGranturismo.net ..
Holden Commodore SS '04 - 455pp, 334 HP, 1658kg
Holden Monaro CV8 '04 - 452pp, 334 HP, 1658 kg
Mercedes CLK 55 AMG '00 - 464pp, 353 HP, 1620 kg
Mercedes E 55 AMG '02 - 489pp, 477 HP, 1910 kg
Mercedes SL 500 (R129) '98 - 429pp, 307 HP, 1800 kg
Mercedes SL 500 (R230) '02 - 445pp, 307 HP, 1840 kg
Audi S4 '03 - 463pp, 345 HP, 1660 kg
Audi RS 6 '02 - 482pp, 468 HP, 1840 kg
Aston Martin V8 Vantage '99 - 502pp, 553 HP, 1970 kg
Jaguar S-TYPE R '02 - 474pp, 406 HP, 1800 kg
Jaguar XFR '10 - 488pp, 513 HP, 1960 kg
Jaguar XKR Coupe '99 - 468pp, 377 HP, 1640 kg
Jaguar XKR R Performance '02 - ??pp, 408 HP, 1735 kg
Jenson Interceptor MkIII '74 - 466pp, 392 HP, 1814 kg
Chrysler 300C '05 - ??pp, 346 HP, 1878 kg
Chevrolet Camaro SS '00 - 451pp, 325 HP, 1560 kg
Chevrolet Camaro Z28 Coupe '97 - 434pp, 290 HP, 1561 kg
Chevrolet Corvette GRAND SPORT (C4) '96 - 458pp, 336 HP, 1496 kg
Chevrolet Corvette ZR-1 (C4) '90 - 467pp, 386 HP, 1600 kg
Pontiac GTO 5.7 Coupe '04 - 464pp, 356 HP, 1690 kg
I'm sure there are a few more besides that but those are the ones I have so far, judging by the weights I think 1600kg isn't a bad number to aim because there's only a few cars that would need up to 104kg ballast added and the rest should need just a stage 1 or stage 2 weight reduction which I think is a similar spread to the 1300kg class for the other event.
Again just going on those basic numbers I'm thinking that maybe 550pp and 550-600hp could be around the general area to aim for, maybe even 600 for both but it'll become clearer once we get some actual testing number put up
.