Reg sleeve flex, I calculated a number years back for a thread on this same topic, and it was something like the sleeve would have to bow a 1/4" (???) to get a change in toe of .030". I don't see how this "phantom" flex on OUR cars is an issue worth much discussion.
Check that calc. I may have it somewhere myself. Maybe later I'll remember where..
The reason I suggest double checking is that one quarter, and certainly one half turn on the tie rod sleeve makes a measurable change of the toe.
And a 1/2 turn shouldn't be 1/4".
If my calcs are correct 18 TPI would result in .11" change in steering linkage for every full rotation of either sleeve.
The resulting toe change will be some ratio larger than that.
All that aside, IMO the main reason for larger sleeves and tie rods were (a) endurance especially for high shock load on the ball joints (b) maintain a stock appearance.
Plenty of classes where the rules didn't or don't allow welding of suspension and steering components.
There was a time when even sub-frame connectors were not allowed to be welded in SCCA Street Prepared. My recollection is that welding the sleeves may have been a no-no as well. I was already out of catagory due to the frame connectors, so welding the sleeves was an easy decision.
The only caveat to welding the sleeves is keeping splatter off the threads - especially the lefties. Not a job for any old hack welder.