Hi, I don't know the Heliopan pol filter but I got for the lee filter holder a Sigma EX circular PL filter in 105mm (it's not a slim version). I use it for about a year know and I am satisfied. The effect seems to be a bit less strong than my other pol filter. But it was the only effordable 105mm pol filter I could find. I paid last year 130€ for it at Amazon Germany.
I haven't tested it an UWW for vignetting, yet. Until 18mm on full format it should be OK with 2 filters slots in the lee holder.