I have a big head and am literally right on the border of the FAST helmet sizing for L/XL at 23-1/4 inches head circumference. Should I go for an L or XL? Plan on installing 4G zero G pads if it makes any difference.

Get bigger than you thought you’d need and remember that the 4D pads are thicker than what they come with.

Always go up with Ops Core. I was right on the border too and tried a L to keep the profile down and it was too small. XL feels great. Also would recommend trying the stock setup before switching pads. The older Ops Core suspension systems kinda sucked but the Fast SF is super nice.

https://www.ar15.com/forums/armory/Physical-Size-Difference-between-Ops-Core-FAST-SF-Large-and-Extra-Large/18-536076/

I asked pretty much the exact same question several months ago. According to the YouTube video, go with the larger size. You can remove the screws and adjust the internals to make it smaller if you need to.

Thank you for the feedback, I will go with an XL. And will search threads related to the topic before making a post in the future.

On the Fast SF,  botach has them for $1300. Is there something I'm missing or are these legit? I just got a TW exfil sl and its really nice but the AMP helmet mount adapters suck and make it a deal breaker.

Historically, Botach has been somewhat deceptive in their advertising. I'd certainly look elsewhere to be safe. A Member I know ordered a high end helmet and received something totally different. It took intervention from the manufacturer themselves for him to get a refund. Botach was resistant and argumentative despite the clearly obvious item being different than shown on site.

Your TW really should accommodate AMPs well. Perhaps go with some different mounts for them. The Comtac's I use on my TW work absolutely fine w their Quick Attach Mounts (Not saying get Peltors!)

I'm sure someone here with AMPs and a TW Lid can assist you with some different ideas.