I normally run an extra 5-10psi at Herne Hill. Tubs stay at home, but I killed a couple of tubeless tyres with sidewall gashes before I started pumping them up harder. A bit of extra sideways action is a small price to pay for making it all the way to the finish in one piece...
Sneakily != illegally
It just means don't tell / show your mates because they might get annoyed or do the same resulting in some weird tyre size arms race