The main reason i think Cars are soo much slower then last year isn't just the downforce loss its the massive gain in weight over the previous season, at minimum weight they are close to a full tank of the V8s, i think if you had a V10 Era car at the same weight as the V6 Turbos it would be close, but the V10s have that extra grunt, ricciardo got 362km/h at Monza last weekend which is around 10km/h off Montoyas top speed in 2004, but that was with DRS which would add quite a bit of speed.
I think Next years Engines will be developed to the point of Surpassing the V10s Power though, as you can see they have plenty of hardware upgrades already lined up for Winter.
Also 2004 allowed tyre changes your thinking of 2005.