What's the best oil/ weight to use

I've got a 2013 roadster that was originally running 10 W 40 oil castrol
I'm now running 10 W 50 oil in the machine and although running well -doesn't seem to have the same grunt as before.
Does running at a heavier weight oil do any long-term damage to the engine as I'd like to revert back to the 10 W 40 and re-gain the horses lost.

The oil could not make such an effect on performance. Even 'seat of the pants',.
Oils will make a big difference to transmission and gear change feel though.
Most high grade full ester base synthetics are fine but cheaper semi synth may be noticeable.

Triumph spec is full synthetic Motorcycle only wet transmission oil 10/40 - 10/50.

You cannot use friction modified or car oils which are usually friction modified.
Mineral based oils are also out including motorcycle specific Castrol 4T versions, they will run OK but not optimal.

If you do a lot of summer running or ride in the tropics 10/50 is ideal otherwise overkill as the radiator keeps thinks a lot more heat regulated than aircooled motors.

Castrol Power 1 20w-50.

I agree with @Ishrub regarding anyone's ability to feel a performance difference just by switching engine oil weight. I also agree with his statement regarding the oil quality/type causing you to notice a feel in tranny shift characteristics. Some oils will cause your shift experience to feel snatchy (if you know what I mean) verses silky smooth. Use a quality motorcycle specific full synthetic oil in the weight Triumph specifies and you shouldn't have any problems. The last thing you should do is use any oil for automotive use which has friction modifiers (most car engines). Those friction modifiers will cause your clutch to eventually slip.