methinks you are expecting Too Much. it is not a great help with sil-nylons, we know that. you'd think it ought'a help. but it don't.
i suspect that since the stuff is so silppery, that in the event that the lower foot (metal grippers) can;t get a hold on it, why should the new top foot (much cheaper a unit and kind of sleazy plastic grips) work any better.
that's my spin on it.
but dang, they're hella fun ! (easily 15 bucks worth)
you can watch them do their .. whatever they do... and that's cool.
i further suspect that a real walking foot machine would be a much nicer animal, being as it's built to do what it does.
as far as power goes, maybe you can get under your machine and clean and lube as necessary to reduce friction, which might sort of boost rpm's and feel like more oomph. a new drive belt can work wonders in that regard.
i have had more good'r luck with either a teflon foot (nice) or a roller foot (better, but hard to thread)
i simply rubber cement my sil-nylon for the time it takes to run it thru the machine and that works fine.