I just pitched onto yest cake with about 2 inches of beer on top of the cake (my autosiphon crapped out on me). I read the thread about why not to pitch onto cake and decided to give it a go anyway. I'm a noob and I was curious how it would turn out.
I put my wort right on top and was expecting a rocket fast start and/or massive krausen.
Honestly, I didn't notice the difference. I can see through my lid on my plastic bucket and I often watch to see how the top of the beer looks, looking for signs of the gathering storm.
I noticed between pitching dried S-04 and on top of the cake, it was about the same start time. It's in full swing now and the krausen is decent, but not volcanic.
Will I do it again? Nope. From now on I'm harvesting my yeast for washing. Since I'm leaving for the States in a few weeks, and I was curious, I went for it.
It'll be really interesting to do the same brew when I return (all grain of course) and pitch some fresh/washed yeast and see how they taste.
My 2 yen (no cents here).