I have recently put my MK4 back to standard from some slight mods, as my renewal this year was £450 tpft  

  i'm 39 yo with 9 years no claims. 
I had RS6 reps, buleflame cat back with 4 mo valance, lowered 35mm, R32 replica side skirts. 
Nothing was done to improve the engine output, it still put out a poxy 150 bhp.
I am now insured with more than for £180 fully comp. How on earth can those mods more than double my premium ?  
 Out of interest i did a quote for an anni. £250 fully comp. ( more power, better wheels, better exahust and bodykit as standard ). Where is the logic ? 
I am sure insurers are using modding as a cash cow just to rip us off.
Rant over. I am now an oem freak