Last year, my modifications cost me about £250 - £300 on top of my £450 at Adrian Flux (something to do with an MS90 conviction kinda worked against me too)
This year, just before renewal date, I had a flyer from Liverpool Victoria with a requite that looked a few quid cheaper than my renewal.
Stroke of Luck #1 - the quote only gave me 3 years NCB - I have max, so quote dropped by about £100!
Stroke of Luck #2 - I mentioned my extensive mods - they asked me to list them all (just as well I'd written them down, I always miss some when I try off the top of my head)
Big turbo
Uprated Intercooler
Chargecooler
Chip
Exhaust
Boost controller
Fuel pump
Air Filter
6 speed gearbox
Forge Dumpvalve
17" ProRace1 alloys
2 strut braces
Uprated discs & pads
Uprated antiroll bar
Uprated shock
Uprated springs
Extra gauges
And how much did that little lot add to my cover?
Nowt

Apparently, due to my "mature years", the modifications incurred no loading, as I'm apparently far more sensible than the normal perception of "modifications", which tends to comprise a snow-plough front bumper, 19" alloys with Taiwanese tyres and three grand's worth of ICE.
"ageist"? - yup, and lovin it!