Well, the good old XT got me to work and back this week without drama. It started when I asked it to; it ran properly; and it didn't break down. The engine is also noticeably smoother (which I put down to cleaning the camchain adjuster) and perhaps slightly more powerful (which may well be the result of cleaning the valves and piston, and lapping the valves in properly, as there was evidence that one of the inlets was not seating).
One small concern is the behaviour of the bike when you shut off the throttle. Normally (on all XTs I have ridden, and on this one up to now), shutting off the throttle causes a massive amount of engine braking - so much so that, in normal riding, use of the brakes is quite a rare event. This engine braking seems to be severely reduced, to the point where I am using the brakes far more than usual. Since the brakes are fairly crap since I fitted the new pads (off eBay, never again), this is a cause for concern.
Also, the idling is not the steady and reliable thing it used to be. Whether it had been ridden ten yards down the drive, or fifty miles at top speed, stopping and idling just meant an immediate and steady thump-thump from the engine, and a crisp response when the throttle was twisted. I've had so many bikes in the past that didn't do this, that I really valued the reliability of it. Now, it is stalling occasionally when I release the throttle, and the idle speed is a bit variable, both with choke and without.
I did take the float bowls off to check the float heights when the carbs were off, but I don't think it is that. The only significant thing I did when I had the engine apart was to dismantle the rather complex throttle cable arrangement, which uses two cables, one to open and one to close. They are adjusted by a rather crude arrangement of locknuts and screw barrels. I suspect that I have not adjusted this back to how it was.
It is unlikely to be tackled today, as it is pouring with rain outside and blowing a hooley, but I now have some clear tubing to rig up a temporary fuel tank, so that I can make all the adjustments in real time, and not have to put the fuel tank back on after each twist, turn and nudge.
If tomorrow's clear, that's where I'll be.