A wheel is a wheel, believe it or not. Saving another 50% of your initial budget to buy a wheelset that will have a negligible (if any) performance benefit is a daft idea.
THe problem with wheels and frames and whatever else, is they feel different/good for the first ride, and then they feel normal after that. Granted, there’re some that’re way to heavy or flexy, but if you’re spending £350 then this’ll be avoided.
I would agree with the suggestion of good tyres.
BTW, Ksyriums are good wheels, but I’d just buy something cheaper. The fact that you’re riding a Defy suggests that all out performance is probably not at the top of the list.