Sounds like doing a dodgy to me - Why risk yourself running stuff you shouldnt .. If you have an accident - and the insurance company doesnt know whats in your car they will screw you over - big time
On top of that if your car causes an accident - and you cant drive it home - it will be looked at most likely - once again - no declaration of turbo - your fecked
In an ideal world you should look at noting the turbos and stuff - least you know your covered - if the premiums to high for a turbo at your age - well , either suck it in - take off the turbo's and waitr till your eligible for cover - or find a insurance company that covers you and pay for the priviliage.
I know personally - that my isnurance company covers any modification imake to my car as long as its legal and declared ..
I drive a ta22 - with a sligtly detuned drag motor in it - with suspension mods , seat mods - and other bolt on engine mods and i haver no issues at all for cover, i just call them up - say ive done say a carb modification , or camming, they check their books to see if it needs engineering aproval - if so i get a plate - if not - they note it on my policy and off i go ( note im with shannons - 23 yo with a good driving record, and one claim )