Erg...the pain of fitting an aftermarket (or third party) piece. Looking at this logically, if the factory tabs fit with your factory soft top and fit with your other hard top, I would suggest that if any modification is to be made it should be to the aftermarket top. I say this because any alteration to the OEM part to accommodate the third party part will, in turn, cause issues when reverting back to the OEM (I hope this makes sense). If you do decide to alter the OEM, I would think it wise to check what the cost would be of an OEM replacement (as Porsche parts can be expensive).
Whatever you do, have fun....after all, this seems to be turning into a hobby for you.
