Thought I'd let everybody know, as I'm sure there will be other people interested: Unfortunately they will be dissapointed.
In the end it was probably one of these things:
•Smooth FPS seems to cause big slowdown (Minecraft Optifine setting)
•Smooth world also does (Minecraft Optifine setting)
•Possibly a reinstall without mcpatcher, which may have been conflicting with optifine, when running HD textures.
FYI, interestingly when in full screen the GPU shows 25% activity (even in the prior situation with poor fps), and about 50% cpu activity, so there don't seem to be bottlenecks.
Also, advanced opengl seems to drop the fps.
FYI, I now run render distance far with Vsync on at 75fps and hd textures.