Easy enough to track engine temps if you have the right tools, since the data is available from the ECM and can be pulled up with various scan tools and things like a Power Vision. All you have to do is find a large group of Harley owners, half with Twin Cooled models and half with oil cooled, have all of them install a PV or similar device to provide engine temp readings, and then have everyone set out on a varied ride including long periods of sitting in traffic, low speed cruising, and highway cruising. Each rider will need a passenger to write down readings, and all passengers will need to be in voice contact so they can write down readings at the same times. Or everyone could just invest in logging equipment to automate the data collection process.
Odds are that in highway cruising either system would be OK. Once you get to low speed and no speed situations, there is absolutely no doubt in my mind that the Twin Cooled system will maintain a more consistent head temperature.
If you want to blow a bunch of cash to convert from one to the other, go for it and report the results. If you're waiting for someone else to run a statistically valid test and report the findings, I think it will be a long wait. And I'd have to have more than just a few opinions without supporting facts to blow that kind of money myself.
JMHO - Jerry