It depends on the IV distribution, but the difference between 90% and 100% mon is pretty minuscule in general and unlikely to matter often.
Lvl 40 mons against lvl 40 ZH/DG Blissey:
Machamp 11/15/15 Time to win 51.6s, power 94.7%
Blaziken 100% Time to win 54.6s, power 83.4%
Machamp 9/10/10 Time to win 51.7s, power 84.1%
So Machamp can be way below what most people consider acceptable IVs for maxing and still come out ahead of Blaziken. This will always depend on the matchup, 15a Blaziken will reach Counter breakpoints sooner than Machamp, which will result in a big boost when it crosses the breakpoint but Machamp doesn't.
Hariyama and Breloom don't have an exact point where they become better than Machamp due to their stat distributions. Against the level 40 Blissey, level 40 Hariyama deals about the same DPS as a a level 20 Machamp while its TDO is somewhere between level 30 and level 40 Machamp. Level 40 Breloom will deal almost as much DPS as level 40 Machamp but its TDO is a bit below level 30 Machamp.
EDIT: Pokebattler was lagging and I looked at wrong movesets by accident. L40 Hariyama actually has more power than L40 Machamp and is only a bit behind in DPS against L40 Blissey.
Leveling up higher can absolutely make the #2 mon surpass the #1 mon. Sceptile is very close to Exeggutor in DPS and will surpass pretty easily if leveled higher. Venusaur is tankier than Eggy but further from the DPS so catching up is harder.