So how do the +P 45 loads size up against the 10mm? I am pretty much decided on an autoloader, but, as stated earlier, can't afford a G-20 right now.
To answer the question, they don't.
To compare apples to apples, you have to compare sectional densities, not bullet weights. Because we want lots of penetration against critters, we want the highest sectional density we can get.
A 230gr .45 has about the same sectional density as a 180gr 10mm, and a 200gr 10mm would have about the same sectional density as a 250gr .45, except that you can't find 250gr .45 ACP ammo anywhere.
The hottest .45 ACP+P stuff I've seen in 230gr maxes out at 950 ft/sec for about 460 ft/lbs.
Cor-Bon's 10mm 180gr JSP is 1,320 ft/sec for nearly 700 ft/lbs. Their 200gr Penetrator is 1,200 ft/sec for 640 ft/lbs. Some smaller brands (e.g. Texas Ammunition Company) offer even hotter 10mm hunting loads.
Even .45 Super isn't really better than 10mm. TAC's .45 Super Express will get you a 230gr bullet up to 1,200 ft/sec... but that's from a 6" barrel, and the Cor-Bon 10mm figures are from a 4.6" barrel. With 6" of barrel, the Cor-Bon is going to be knocking on 1,400 ft/sec, at which point the whole thing becomes a wash at best... except that the .45 ACP+P is probably still losing out in raw penetration ability to the 200gr 10mm out of equal barrel lengths.
Of course, once you get into handloads, all bets are off, but breaking 800 ft/lbs with 6" barrels and heavy bullets in 10mm is very do-able if you are careful and use the right powders (e.g. 800-X). You really can't reliably beat the ballistics of 10mm in an autoloader until you get to stuff like .460 Rowland or .40 Super... at which point you are probalby better off with a revolver if you want a decent service life from your firearm.