Factor in the cost of insurance as well (although I don't know who anybody could tell if you had remap).
The great thing about stage 1 is that you do not need any supporting modifications. Having said that, this is the beginning of a very slippery slope...
It's 2016 mate, insurance companies are well aware of remapping (and cars that are commonly mapped) and if they really wanted to find out they would use their resources to do so. If there was a nasty bump and someone was hurt it would probably be one of the things they'd go looking for to avoid paying a claim, and there could be even worse repercussions following that.
There's a code on the ECU (on VWs at least) that comes up if the car's software has ever been rewritten, think it's TD1. It's pretty much a big red flag.
I'm with Adrian Flux and the increase in premium for my remap really wasn't much at all. Worth just factoring it in to the cost of tuning like you say, for peace of mind.
I've been thinking about that recently. I don't condone non-declaration of maps, but for an insurance company to prove your ECU software isn't standard - and also prove that you didn't know it was mapped when you bought the car - would be difficult?
In the MK5, the only clue is the 'flashcount'. If it's greater than 0, it's been flashed. They can't decline the claim based on a reflash showing on the counter because it could be a perfectly innocent factory recall map. They would need to interrogate the ECU software, which would mean needing programming software for every ECU make and model out there. And they need to know what they're looking for because it's all in Hex.
TD1 only comes up on the newer MK7 ECUs, but even then people are circumventing that by using a tuning box or piggy back ECU. The internal software hasn't changed, so the ECU is none the wiser.
So yeah, tell them for peace of mind, but at the end of the day I think it will be very hard to prove. I think that's why the small print was changed recently to "To the best of your knowledge, is the car standard?". Someone who knows nothing about cars could buy a GTI and be none the wiser!