At 500-800 yards you are going to still be supersonic AT SEA LEVEL with most factory 175 gr match ammo out of an 18" barrel. If you are in Colorado at 5,000 feet, you are going to stay supersonic maybe to 1000 with that barrel length, again with something like FGMM.
If that's all your shooting, yeah dial in a tiny bit more elevation and there will be no difference. If you are trying to hit that 1,000 yard magic number, well, things change:
This is generalizing a bit, but to keep it simple, a rule of thumb is that to keep a 175 grain SMK from going transsonic at 1000 yards it has to start out going 2600 at sea level. That is what Federal Gold Medal Match 175 gr starts out at out of a 24" barrel. Again, at sea level, from a 24" barrel, that load is barely staying supersonic at 1000.
Note--again, if you are shooting at altitude, don't even worry about this shit. You can start the bullets out maybe 100fps slower and they will still say supersonic at 1,000.
But at sea level, an 18" or 20" barrel using your most common factory match ammo, FGMM 175 grain, is NOT going to keep the bullet supersonic at 1000. It will prob be supersonic out to 900 yards. Yeah the bullet will land and yeah you'll hit the target some but your hits will be noticeably less reliable than at say 900 yards.
Again, if you handload them hotter, this isn't a problem. And I've heard that the new Corbon 175 SMK load is running 2740 FPS out of a 24" (ie way hotter than FGMM) so if you are running that ammo, you should be fine at sea level out to 1000 with a 20". Maybe an 18".
This is based on my own experience running my 20" bbl shorty at 1,000 vs my 26" bbl rifle (both 308) at 1000. Hits with the shorty at 1,000 yards are noticeably less consistent than they are at 900 yards. You can tell the bullet is starting to act goofy. Whereas the 26" bbl stay consistent to and past 1,000. Again, this is at sea level.
Hope that makes sense. Maybe you already knew it.