Nice car, but I don't get the 'sleeper' bit
A sleeper/Q-car is a totally normal and boring looking car/shed, which has been souped up to outperform most on the road.
This comes from the original idea from WW2 of British Navy 'normal looking' merchant ships, hunting for subs and looking all vulnerable, and then when they found one, they'd drop the normal/boring sides, revealing high powered weapons, which would be used against the enemy.
In the same way, a true sleeper on the road has to be standard and boring looking, i.e. not looking like it can shift at all, then it really surprises you by leaving you in the dust!
There was a transit van on Top Gear like this once, American bootleggers who built sleeper hot-rods did, and some other people have built true sleepers since, but not many.
Your Leon is anything but 'boring and slow looking' - it has a private reg, uprated wheels, a tuning
sticker in the rear window, debadged, carbon detailing on the outside, gleaming polish/shine, uprated exhaust, etc, etc....so it's no sleeper, but it is a nice Leon