More different than you'd expect. My Westie is pretty track-focused, so I'd assumed that it would have at least given some preparation, but short of just being familiar with driving a quick, screenless car on track, there wasn't a massive amount of directly transferrable experience.
The first and most apparent difference is in the driving position and controls. In the FF you're virtually lying down, cockpit in very "snug", with limited head and elbow movement, steering is outragously quick (3/4 turn lock to lock), and the gearstick is on the "wrong" side.
Obviously, the FF has a lot less power than the Westfield (105 vs 225bhp), but between gearing and weight (420kg vs 560kg), you don't really notice a massive difference other than at the very top end.
And it doesn't handle like a Westie (well, not mine anyway). I've got the Westie set up to be quite playful and fairly tail happy, which means you tend to muscle it around a lap, dealing with oversteer as it comes, and using the surfiet of power to mop up any less-than-perfect corners. Not so the FF - you have to keep it all very neat and tidy, and get every braking point, entry speed and corner line right first time every time. It will tolerate a degree or two of yaw, but much more and you're definitely playing with fire - it snaps very quickly. Plus, you can't take a second bite at things, the flywheel is so light and the powerband so narrow, that if you have to lift for correction, the engine isn't ready to immediately pick back up and you lose loads of time.
In terms of outright pace, my best ever dry lap of KH in the Westie is top 57s, and my best lap from Saturday in the damp was a 1.01 in the FF, so I reckon in similar conditions, on fresh tyres and with a few more FF miles under my belt, there probably wouldn't be much in it, maybe .5s in favour of the FF.