There is some torsion on the rear block system, the loads are skewed to one side of the blocks a little more than the other. but its pretty minimal and is no more than you would expect from a system that has these loads and hence these blocks.

The blue line through the orange block is there to stop the mainsheet ever popping off the sheave and to stop the block spinning in the boom. it is only held in place by a bolt passing right through the centre of the Karver block.

The small diameter line on its own 2:1 system is a sheeting gauge, it has a shock chord attached to the line and then to the front of the boom, there is a scale marked on the boom about a foot from the font, so you can see how tight the sheet is without looking to the back of the boat. Euan (my crew) wanted it and it doesn't bother me so we did it.

I think the T with the carbon mast is quicker but it is very marginal in most conditions. I think it would be impossible to give it a hard number in terms of increased performance. Certainly in big wind and waves it is much nicer to sail and is for sure quicker. The change to the carbon mast was never for performance, more for consistency in rigs. in this regard it is perfect.