As Glider says, heat on an air/oil cooled machine is based on the state of tune, frictional losses and design to shed the excess heat not used for combustion.
That said, HD has evolved the V-Twin to close to the limits an OEM can do on a mass produced machine AND meet EPA standards, and all the R&D to qualify said product. We may bemoan the MOCO for it's practices...but most other manufacturers of air/oil cooled machines stopped because demand did not match the R&D dollars to produce a competitive product.
Sure plenty of aftermarket product designers like S&S and others can squeeze more...but at a considerable increase in price and sacrifice to some degree the longevity of the base engine unless all the internals are redone also. Overheating as Smitty says is a bit overblown...lots of R&D has gone into heat management issues, and they could not afford to "blow it" and send defective machines out that would bankrupt them in short order.