I took the pad off today and happily it came off with no marking what so ever. It was a clean peal off and I had removed the plastic film prior to insulation.
My GB5 Paste Shim only is very close to what it was with the pad and shim, I would have scores over 4100 but many just over 4,000, I scored 4039 and 4024 paste shim only. You could say from the highs and lows it seemed equal. The bottom case felt cooler than with the pad attached.
The Cinebench R20 also scored about the same as when I had the Pad with Shim, I got a high of 1226 pad and shim and shim only just ran a 1213 1x. But the temps running R20 did hit 100 with out the pad and with the pad it would max mid 80's.
So the tests show the Pad does dissipate the heat better then no Pad, although making the bottom case a little warmer. But output from the device hardly changes from New Paste and Shim vs Paste Shim Pad. It seems to me that Apple has it restricted to not be able to benefit a greater than just the paste alone or just the pad alone. A Apple Governor
My conclusion is if you do the Paste Shim no need to add the pad. If you are not comfortable putting new paste on then Just do the 1.5mm Pad.
I recommend the 1.5mm Arctic pad, not that its the best thermal conductive (not sure that it matters a lot for this mod) but it clearly is a peel off pad leaving no residue or any signs of modification.