Hi Ricardo,
Consider this - One of the reasons the previous owner of my Speedster sold it was that it would overheat on hot days (95 and up) and at idle the oil pressure would drop and the oil light would come on at idle. Goosing the engine would get the light off and at speed the pressure was about 10 or 15 lbs. Not good at all. He brought the machine to my house on a trailer so I wouldn't notice.
The engine was missing the dog house cooler, it had an extra cooler over the transmission, and an oil filter had been installed along with the extra capacity oil sump that you are considering.
I installed the dog house cooler, changed the cooler over the tranny to one that has a fan (comes on at 180), removed the oil filter and the extra capacity sump. The result is that on a 100 degree day I can drive all day long. My oil pressure is 60 lbs on start up, 30 elbos when driving and 15 at idle.
Here is the part you should think about - for me 3 qts is perfect, more oil means longer warm up time, I want my engine to warm up fast and maintain consistant operating temperature, the sump can also reduce your ground clearance. I don't want an oil filter because I change the oil every 1500 miles.
I've owned many VWs and have used external oil coolers on a bus, bug, and squareback, I did lots of freeway driving. Previous bugs and a K-Ghia were stock. All the cars were on a 3k oil change cycle.
I don't think more oil is the answer to over heating problems.
Michael