I had prepared a reasoned reply, but before posting I decided to have a look at the Hunter 260 section of this website. It appears that the boat has a very specific cabin- top jib sheet fairlead setup design. And the shrouds are quite far aft of the mast. From the pictures I looked at, it doesn't look possible to use the existing fairleads for a much larger genoa with the sheets routed outside the shrouds. And inside, the sheeting angle probably would be wrong for a larger sail. You would probably need to install a second set of jib fairlead tracks on the deck (and not much room there) to allow good sheeting angles for a large genoa. The boat is a trailor-sailer (looks very nice) and hence likely to be more lightly constructed in certain areas than a heavier/larger boat. So consideration should be given to changes and location of stress forces on the hull/deck/standing rigging/mast. An idea would be to contact other 260 owners for their opinions and see if/how others have flown large genoas.
Anyway, for what its worth here is what I first drafted as a response:
You may lose some pointing ability, but it shouldn't be a radical difference. Irrespective of pointing ability, with a large head sail (genoa), I don't think you have any option but to route the sheet outboard of the shrouds since the clew will extend aft of the shrouds (at least the forward shroud anyway). If routed inside the shrouds, any wind velocity at all will press the sail and/or sheet up against the inside of the shrouds causing shafing. And when you want to sail off the wind and need to let out the jib sheet, the sail will have an awful trim as the sheet will be bent at a fairly radical angle around the shroud.
My boat has the shroud-to-deck location only a few inches from the toe rail. The boat was designed to use the toe rail as the "jib track" with the sheets routed outside the shrouds. For both my 135 genoa and 95% jib, I have rigged barberhauler arrangements so that I can draw in the clew/leech all the way to the shrouds. This does improve pointing ability and speed when close-hauled to the wind, but I think only just a bit. Its not a radical improvement. I don't think you will loose 10 degrees by routing the sheet outside the shroud.