If SF can have their drop in make good metal to metal contact with the head then the head will be a fairly massive heat sink. I do not have the GPL drop in, but I am trying a DX drop in. Using a yellow Nitralon body and a 6P current style head, I am trying this out. There is no good metal to metal contact of the drop in to 6P head, only up front at the window and an O ring. After 4 minutes head is getting warm. At 12 minutes head is a little warmer. Light output (bounce with lightmeter) is not dropping significantly. Stopped test at 20 minutes. Head is much warmer though no where near hot. Light output did not drop drastically. I will do this same test procedure with an all metal 6P later, and report back to this post.
Again, if there is good metal to metal contact with the GPL drop in with the metal bezel of the newer G2L then the head will act as a good heatsink.
Bill