Had a heavily modded MK4 (TT suspension, LSD, GTB2260 with supporting engine mods ect) insured with Adrian Flux for a number of years and the policy increase between declaring all the mods was approx £50 to £70 for the year.
If you're involved in a serious accident with life chancing circumstances to the 3rd party, the police/insurance would possibly want to inspect the vehicle afterwards. You'd possibly be slapped with a large fine, 6-8 points on your licence, possible driving ban as your insurance company will deem the vehicle not insured as you haven't declared the mods.
Might end up having 3rd party solicitors chasing you for injury claim,vehicle repair, loss of income, courtesy car and leaving you in a life changing situation which might not have been your fault at all..
I've done it before yes, but thinking about the consequences changes your mind very quickly.