First of all, how the teams would do against a 5A opponent isn't relevent. Its likely that SLC has as many people participating in athletics as Celina has in their entire school. 2A football teams have about 20-25 players suit up for varsity. SLC has probably 50 or more.
Second, SLC hasn't had a lot of solid division I talent there. There are few, if any, people you can name that came out of SLC that are current major conference all-conference. About the only somewhat big-name player is Garret Hartley of OU, and he's a kicker. Chase Daniel at Mizzou may start there, but he won't be a factor in the Big 12. Other than that, there aren't many DI players out of SLC.
In reality, the largest Texas high schools have over 5,000 students, and two of the biggest: Plano and Plano East, have that many on their campus which is entirely Juniors and Seniors. 2000 is barely enough to make 5A. In fact the cutoff this year was something like 1978.
The point I was making was that playing in 2A, McCoy did not face the level of competition that 5A players face. I'm not dissing small town football, I've been covering it professionally as a photographer for the last four years. I prefer going to a small town game over larger schools. However, I've also been covering college games from the sidelines (Division II). The increase in player size is astonishing. Speed is also significantly higher. I don't think we have any disagreement, but maybe I phrased my original post poorly.
BTW, one of the things that's kind of funny is that on a full week I do the 7th grade, 8th grade, and JV on Thursday, the Varsity on Friday and a college game on Saturday. At 7th grade, the field seems huge. Often the refs, coaches (and photographers, heh, heh, heh), will cheat in five or six yards from the sidelines. When a kid runs out of bounds, you put out an arm and catch him. By JV, you move to the side, cause they're too big to catch. With varsity players, you learn to duck and move, cause those guys can break bones if you're not ready. On the college teams, stay five yards off the sidelines, and they'll run over you like an eighteen wheeler hitting a Toyota.