I actually did a side by side comparison. I upgraded the backup P2 first (foil and antenna) and the compared to the main P2 (stock antenna, no foil). Definitely an improvement. It doesn't make it perfect but if you find yourself waiting a long time for sats, it can't hurt.
Aye, there's the rub. Only anecdotal evidence that the foil mod actually helps. But, the next time I have the shell open I'll try it since I have a couple of dozen flights logged from my location and I have never seen more than 7 sats at takeoff and 11 in the air. If I average +1 over time, then I will declare it a "fix". I suspect that it's of more advantage to the Vision models because of interference from the 2.4 GHz WiFi module.