You really should review the published materials on charge pumps.

The caps on pins 2 and 6 are output reservoirs, not just decoupling cap's, and not subject to the all-AC behavior of the ones between 1-3 and 4-5. That's why many applications still use the original 1 uF cap that was recommended with the orignal Max232. The later versions use 100 nF, IIRC, for the pump cap's.

If you reference the V+ to Vcc, there need be a smaller charge increase on the cap than if it's referenced to GND. The pump is AC-coupled to whichever reference you use. It has no contributory path from Vcc. The pump has to work harder to maintain +10 volts with respect to GND than to maintain +5 volts with respect to Vcc. The weaknesses of the capacitor are also worsened by the larger potential across it. It makes perfect sense that Vcc would be better than GND as the reference.

I've never strayed from the original hookup. I've used other pin-compatible devices, but they all seem to like the Max232 hookup just fine. All this becomes more apparent with the lower-voltage devices, e.g. MAX3232.

RE