well wonder no more :

Good Tracks: Loss of 2.2% and 14.98% Strike Rate (16515 selections)
Wet Tracks : Profit of 5.8% and 14.81% Strike Rate (7379 selections)

Used Aus tracks only as Wet is very different in different countries.

So there you go top weights in the wet actually produce a profit because like you most people percieve the wet track hurts them but in reality its only a 0.2% difference of 1 - 2 horses per 1000 top weights which lose compared to the dry tracks.

But hey keep it a secret ... don't want others knowing these important stats.

Betfair SP prices used for analysis.