I have mine with Classic Cover, they've always been good but I've only ever claimed a windscreen.

You might want to phone tower and ask them if your car is covered. Tell them the mods, exhaust and bov, pretend to be a prospective customer.    It pays to find out before you have to make a claim, touch wood, than when you do and realise you aren’t entitled to be paid out.    Tower is pretty good, but they’ve toughened up the criteria in the past few years, as have AA. Im with AA and being a larger insurer with a solid credit rating and high volume, there’s less finding ways to not pay out a claim, and more getting claims sorted.    Plus you find out who is a good insurer when you need them - almost 10 years ago my MR2 got broken into, AA took care of the tow, repair, and were really efficient. AA I believe were also one of the few if not the only insurer that sent people on the ground in Christchurch after the earthquakes where other insurers stuck their heads in the sand. 

Kinda an age thing but they do insure other vehicles that don't quite fit the standard daily driver criteria.  Mines a 2010 but heavily modified and does barely 4kkm a year

@Timmah will have a look at them. Thanks. 

I'm with ClubAuto, and I know no better insurance wise. GTi and 2002 WRX with them.