Foggy,
I bought one of these from the same seller.
I fitted mine and blanked off the cam case breather as suggested. However, when checking the oil, I noticed air being sucked down the dipstick tube. With the breather blanked off, there's negative air pressure created within the bottom end of the engine. Whether this would do any harm, I couldn't say, but I didn't like the idea of blocking of the breather, so I fitted a small air filter.
It did make only a minor difference in performance, but at least the TB doesn't distort when tightening up the air filter.
If you're thinking about /are doing The Speed Championship, sprinting, then check with Mark Durrant first, that it would be an exceptable mod, for the class you'd be in.
Cheers, Clive