Pretty smooth looking lap ya got there. I would recommend taking a wider line entering turn 1 to set yourself up better for the Dunlop Chicane. You are in the middle of the track when you start into the left hander. Being more to the right will help the entry there.
With that said, your overall time through Mulsanne Corner is pretty close to what I've been running. The rest of the lap I gain about a second, though. I think what hurt most on this lap was the Porsche Curves like you said. The straight after Arnage, I upshift to 4th and rev the crap out of it (staying out of 5th), then I downshift into 3rd for the first right hander, up into 4th for the first lefty, and back down to 3rd for the final right and left handers. I think dropping into 3rd on the first corner helps a bit with rotation and doesn't bog the car down as much upon acceleration. You also got a little wide on the final right hander in Porsche which made for difficulty in that final left.
Still, not a bad lap at all. You're already making big gains. I don't see why you won't make more before the end of the TT.