I ride the same pedals here in the rainy northwest, and I don't find them slippery at all in the rain. Riv has instructions for adding BMX spikes if you find they slip on you. (I buy my BMX spikes at the LBS for something like $10-15 for a box with more spikes than I'll ever need) They are not as grippy as my spike-y Crank Bro 5050, but they are a long way from slippery for me.
Keen and Chaco sandals have no problem on my MKS Lambda pedals.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson