I already have a gen 3 Glock 34 for Production and SSP.

I want to get a Lone Wolf 20Long upper converted to 40SW to play with, but I am not seeing tons of application for it. It looks like it would be useless in IDPA.  I am not even sure such a heavy long slide will offer any real advantage over a G35.  It might be too heavy for a .40, but I do love the feel of the large frame Glocks.
I could use it with no comp in USPSA Limited and Limited 10
With a comp I could go Open in USPSA, IPSC, and GSSF.
PLEASE CORRECT ME WHERE I AM WRONG AND ELABORATE.  I do not want to buy a useless slide.  I might be better off to just build a no comp G35 for USPSA Limited and IPSC Standard.

I want to know the biggest Glock for each discipline.

USPSA - aftermarket slides and barrels are OK
Production -  G34 legal (ON LIST) with some mods and INTERNAL changes OK.  No reason for .40SW. 10 rounds per mag. Aftermarket slides and barrels OK if they are the same size as factory.  Gun must mostly LOOK factory from the outside.
Limited - anything goes EXCEPT comps and optics.   G17Long and G24 fine.  9mm Minor and .40SW Major Power Factors. 140mm mags
Limited 10 - same as above, but with only 10 rounds per magazine.
Open - anything goes.  G17Long and G24 fine. Comps and Optics needed to be competitive. 9mm Minor and 9mm Major Power Factors. 170mm mags
Single Stack - 1911's

I am guessing a threaded barrel with no comp attached would be allowed in USPSA Limited?

IPSC - aftermarket slides and barrels are OK
Production -  G17 legal (ON LIST) with only internal changes OK.  No reason for .40SW. Must have 5# trigger pull, max 15 rounds, no more than 5" barrel.
Standard - anything goes EXCEPT comps and optics.   G34,35 max due to "THE BOX".  9mm Minor and .40SW Major Power Factors.
Open - anything goes.  G17Long and G24 fine. Comps and Optics needed to be competitive. 9mm Minor and 9mm Major Power Factors. 170mm mags
Classic - 1911's

IDPA - aftermarket slides and barrels not allowed.  BOX determines max gun size (34,35,41)
SSP -  mostly stock G34 legal with unmodified appearance, internal changes OK.  No reason for .40SW.
ESP -  G34 legal with trigger changes, trigger overtravel stops, and lots of mods OK.  No reason for .40SW.
CDP - 45ACP only, so this is mostly for 1911's, but a G41 with no external changes OK

GSSF - Your pretty much looking at stock guns with categories for all models.  The long slides G17L,G24,34,35,41 all shoot in "COMPETITION".  Aftermarket parts put you in "OPEN"

not an answer to your question, but just a comment... I shoot a G34 in Limited minor and 3 gun (it was specifically built for 3 gun)... I previously shot a G35 in Limited major, but even with close to minimum power factor .40sw loads, I can shoot my G34 quicker and more accurately than I could the 35... Production is a 9mm only division, and there is currently no major/minor power factor in multi gun... I have an extended mag button on the G34, so don't shoot in Production... as to my skill level, I've been shooting USPSA since 1982, once upon a time I earned a Limited Master card with a .45 single stack... just some thoughts YMMV

For IPSC production G17, I don't think there are any "internal changes" you could get away with that were not Glock factory parts. Aftermarket sights seem to be OK but barrels, strikers or Gen4 to Gen3 rsa conversion would be a no go in production.

Always keep in mind the first shot 5# trigger pull in production. If you messed with springs and don't have 5#... trouble. Ask me how I know.

If you shoot a G34 in IPSC standard minor you will get murdered by the STI/SVI guys shooting major.

IDPA ESP can't use aftermarket barrels?

My guess is to build the gun the way you want and let them put you in whatever class they say you belong in.

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I did that. Sort of. Showed up with my stock Glock 17, stock mags, kydex carry holster and mag pouches. They put me in "limited" (minor)  Talk about handicapped. 17 round mags and my hits scored differently than guys shooting 22 round .40 caliber 1911's.

Then I discovered production which is where I should have been all along. Everybody has ten round mags. Everyones hits are scored the same.

The only logical reason for a competition gun in 40 is to shoot Limited Major IMO.  A case could be made for shooting downloaded 40 ammo in a 35 with a 13 lb recoil spring, too, as some report it's softer shooting than a 34 with factory ammo..  Otherwise, you'll be more competitive in Production classes and ESP/SSP with a 9mm.  USPSA Open is mostly 9mm Major, 38 Super, 9x23, etc. - small, fast bullets.  I haven't shot Open, but my understanding is the faster bullets work better with compensators?

By and large, though, the person pulling the trigger makes a much bigger difference than what trigger (s)he's pulling.