Hector has said many times that the L36 heads flow better than the L67 (series II) heads because of the placement of the injectors. I would take his word for it since he is doing R&D.

What are the gains and losses from mildly porting a head? In other words, do you gain or lose low end torque, or gain or lose high end horspower?

I have had my heads done and I will tell you from experience, there is a great deal to gain from porting and polishing your heads. First off, GM has done a great job with these heads. But with a stock bench flow of only 180 cfm(intake), they are not really set up for high performance. I had mine done to get 225 on intake....that, matched with the 1.9 rockers and a slightly warm cam, gave me an added 63 FWHP. Just doing the heads (this is a guess, I had both done at the same time) should be about 20-25 FWHP.

The shop I used charged me a little over $650US for the job. I supplied the rockers, pushrods, valves and springs(total-$500US). They did the rest, including retainers and guides. I wouldn't recommend doing just one of these...do at least the rockers and porting. The cam can wait for later...but not much.