What shower do you have? power, electric, mixer etc.. Is it being fed from a combi, kw of the electric shower, what kw is the boiler? Flow rate from hot water storage tank? A few variables to decipher before it can be answered. It probably will be unlikely to alter the performance to any noticeable degree. Also 15mm or 22mm? And are you sure you don't have full bore ones already (some can look very similar to each other). Also what's the pressure/flow rate like for your cold feed?
I didn't think it would make a lot of difference either given its a thermostatic mixer fed by 15mm from a system boiler via an unvented cylinder. Current shower performs fine as its under decent pressure already given the setup. The pipes are inside a wardrobe next to the shower room and I'm changing a few things in there and was considering changing the valves, which are standard, whilst I'm there but its not critical.
If it's a mains pressure system then increase wouldn't be significant I wouldn't think but if it's easy to swap then it wouldn't hurt to move to full bore lever valves, less likely to leak.
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