It has been proven in court that if you buy a new rifle and it is then tested over the limit the punter who bought it isn't to know, nor has the ability to test that. They bought it in good faith that all was above board.
However, if you make changes then you should go and have it tested that it remains under the limit with your usual pellet.
There is an understanding that not all air rifles shoot to the same power with different pellets, nor temperatures, and even the Police now agree. All you have to prove is that with your "usual" pellet it shoots under the limit. If you have made a whole lot of changes then it would be prudent to take steps to test it and ensured that it stays under the limit. Playing the ignorance card only goes so far. The onus is on the Police to show you have gone out of your way to break the law. A 14ft/lbs rifle kind of does if tinkered with. The Police have to make a case now that you intentionally made it all so.
So if you feel like it then do have your rifle shooting close to the limit with the pellet it likes. How close is your call, just don't take the mick.

As important with springers is what power does the rifle "behave well" at? Under the limit. Half my rifles shoot well under at 10 to 11ft/lbs.