Hey ETD. Is it your Sig that is giving you the troubles?
Here are some things to consider.
Regarding the tappet plate: Your plate is probably experiencing too much friction in the mechbox. The modify plate should be able to handle the stress, so I believe that friction is present. The high rof setup increases the ill effects of the friction. Try lubing the plate, but often this is not enough for insanely high rates. I have never tried this but have read that you can take individual ball bearings (just the balls) and drill a few indents in the left and right edges of the plate and insert the balls. I don't remember where I read it but I will try to find the article.
Your feed issues can be many different things. Does it refuse to feed altogether or is there intermittent feeding? Does it feed in semi, but not auto?
In a high rof setup certain modifications can be made to increase feed reliability. I noticed you said that the nozzle is of the "correct" length. The correct length can be shaved slightly shorter for high rof. Never exceed 1mm shorter than stock>very important. Try shaving a couple of tenths of a mm from the high rof nozzle, see if it helps. If it is not perfect, shave a little more. Do not exceed 1mm of shaving or you will start to get double feeds (unless you are cycling at a tremendous rate) and you will also lose airseal with your hop-up chamber.
Anyway, some things to think about. It is hard to tell without more info, or actually seeing the problem is nice.
venture