With regards to the 5 years stretch and examples.
I posted here a while ago on this. I have not yet heard of anyone getting 5 years for an over the limit airgun. However as Terry said I wouldn't discount someone in the future getting it and I only really hear about things happening in my area, so I can't say hand on heart no one ever has.
You will probably remember way back in the late '80s early 90s when car theft - UTMV-TWOC or whatever else you want to call it was the 'en vogue' thing for kids to do. The government introduced a law called aggravated vehicle take and aggravated allowed to be carried. This law also carried a mandatory 5 year custodial sentence. I have dealt with lots and lots of these offences and none of my defendants got that (even on their 5th go!)
The punishments handed out by the courts or 'advice' from CPS never seems to be set in stone. If you have cranked your rifle up to XX ft/lbs with the intention of deliberatelly flaunting the law - or - believing you won't get caught and you do, then I would suggest you may be looking at a bit of 'bird'.
If your rifle has run in and crept up a wee bit over, the chances are it will be returned to you after it has been turned down by a gunsmith. Before anyone jumps up and down, I've known this to be the case a number of times.
Having said that these electronic devices that measure pellet speed are readily available, the majority of gunshops own them. Recently manufactured rifles have anti-tamper devices/stickers in place so it is becoming increasingly difficult to 'blag' your way out of a nasty confrontation with the law. I personally would risk 'getting away' with it on the assumption no-one gets 5 years.
As a side thing, I got myself a brand spanky AAs410 on Tuesday. When I chrono'd it when I got home it never shot above 10.7ft/lb across the charge of air, this was using Bis Mags 10.6grn. I sat down and scoped it up, zeroed and had a major plinking session with it. After recharging the rifle numerous times and nearly a tin of pellets I re checked the chrono, it was doing 11.4ft/lbs. The moral of this tale, don't be tempted to mess with your rifle just because it's low on power when you get it.
Dazz