Hi Wouter, I'm definitely no expert here, but I do have a T4.9 and here are some of my opinions to your questions.

1. Yes, I do think there are benefits of getting a main cut for 1 up sailing. I have always wanted one of these. While I don't think the difference will be mind blowing, I am guessing a one up main would be much flatter and make the boat less powered up so early. As a result you would likely need less downhaul and you'd have more leech tension. However, along with the new main, I am guessing you would need to change your mast setting as well.

2. I think changing the mast setting may make some difference, however, I think a main cut specifically for 1 up would probably have more effect. There is a reason why the 1 up guys in oz have completely different sails and masts for 1 up configurations.

3. I singlehand my T4.9 most of the time using a main cut for the sloop rig, and yes, in cat rig, the boat does not like being pinched at all. I get much better VMG just footing off and going for speed and power. I personally think one of the reasons for this is that the volume of the hulls are made for 2 up, so when singlehanding and pinching plus trying to punch through chop, the weight of 1 person is not enough to force the bow through the waves. The bow rises over the wave, gets blown to leeward and you lose a couple of feet on the crest of every wave. Of course if you're a 100kg guy, maybe this might not be an issue for you, but for a 75 kg guy like me, it is.

4. Solutions? Have you thought just pulling the rotation towards the centre a tad more? This might settle the boat with a little less downhaul. I also tend to sail the T4.9 cat rigged with the traveller off the centre line by about 1-2 inches and while i lose some pointing, the speed is much better. The boat is more settled and you don't need to hal in as much downhaul, solving the problem of inadequate leech tension. The speed then allows the boat to just "lift" to windward even though my bow isn't pointed that high.

Hope this helps