E10 will most likely be fine as long as the carb bowl is drained before any storage longer than a few weeks. The ethanol will start to corrode the aluminium in the carb if left in there for too long. I'll be using E10 in mine and draining before storage. It's worth noting that mainland Europe and the US have been using E10 as standard grade petrol for a lot longer than we have, and I'd be surprised if many people are putting super plus in their garden equipment.
As for performance gains with any sort of premium product, I doubt anyone is near the knock limit of these engines when they're pushing out 50hp/litre. The cooling effect of ethanol will most likely be of more benefit than an octane increase.
