P-99s are just fine as front panniers, I've used them with my Cannondale rear panniers, I don't know the capacity of those.

The other front panniers I use are the Arkel T28's which I do like quite a bit. They are probably 30% larger, and have a very useful top pocket, as well as mesh on the outsides. They have a heavier material and zippers, and a very solid attachment system. I do like those as front panniers. In fact, I would prefer those if you're doing dirt roads, the attachment system is more solid than the P99's.

I don't recommend the Arkel GT54's, they are far too heavy and have too many overly specific pockets.