I have v1 of the PiTFT 3.5 resistive touch screen and have been playing with getting it working with RetroPie 4.3.
These days, the RetroPie image has the overlay so you just need to add the dtoverlay line to /boot/config.txt and get fbcp running (the Adafruit setup script does this fine).
I have found it will not do speed=80000000,fps=60, no way. In fact, it's not reliable at speed=60000000,fps=30. This is corroborated by a comment here: https://blog.petrockblock.com/forums/to ... -retropie/
It seems fine with speed=48000000 and even speed=56000000 but it seems laggy and there is tearing. If people are able to achieve higher speed and less tearing, they are either lucky in their specific screen (tolerating higher speed) or have different settings that they haven't explained well enough for us to replicate.
At any rate, it's a challenge to get the 3.5" display (480x320) working well, as it's pushing twice as many pixels as the 2.8" display (320x240). One thing I'm curious about, is why it's only using SPI, when it is also capable of using 8 or 16 bit parallel data, which seems like it would be faster. Is it a driver issue?