The gaming benchmarks I've seen show a relatively small increase in FPS between the 5300M and the 5500M 4GB, so if gaming is important to you then the <5% increase in total cost could make the 5500M a worthwhile upgrade, but it's not mandatory (unlike the 15", where the Vega GPU upgrade was practically required for all but the least demanding games).
The 8GB appears to make no difference at all for most games and my guess is it probably never will (i.e. any future games which could benefit from 8GB are likely to be so graphically demanding that in order to get playable framerates the graphics settings on this MBP/GPU will need to be sufficiently turned down such that VRAM usage falls below 4GB.
I'm feeling a bit torn, myself. I had initially planned to upgrade to the 5500M with 8GB VRAM for "future proofing," but I'm toying with the idea of just getting the 5300M 4GB. The games I currently play are not graphically demanding, and even if that changes, streaming of video games is about to become a real thing. Plus, there's always the possibility of adding an eGPU down the line.
The gaming benchmarks I've seen have given me the opposite conclusion.