I have consistently modded and mapped cars for the last 10 years and my thoughts
Clutch will be the weak point due to the extra torque and that when you have a mapped car you tend to be lazy and ride the torque curve rather than dropping it down a cog and using the power
The longevity of a mapped car tends to be influenced by how the owner treats it, lest i warm up, lest it cool down, etc etc
The term custom map I detest as not always a plus point for quality and detracts from the custom mappers who are good but you could count on one hand, a custom map will not have anywhere near the level of testing as a generic map from one of the better tuners
My mapped cars have typically done 15-20k a year and some track work, only issues I have had is a replacement clutch on a Scirocco
The mk7 Golf R turbo was an interesting one on the first cars, I together with Revo had cars with this turbo and we did about 15,000 miles between on these, my car never suffered turbo failure and interestingly weight did Revo (and they did more extreme testing) until they upgraded the turbo
My thoughts on mapping the Up are I would like more power but not a huge slug of torque, I think some poor mapping on the torque front will just destroy the drivability
I never map cars I do not own, hence leased R has not been touched, I have not mapped my M2 as I don't think it needs it I value the warranty on the DCT transmission
I have never been a fan of tuning boxes, they only fool things and ECUs can store parameters that indicate where there have been in place (a friend is a VW Warranty Manager), interesting he said it is in their interest to fight for the customer as they get paid for the repairs
I have had a great experience with dealers and modded cars, i generally get my car flashed stock and drive it for a week before it goes in for a service
My advice on modifying is if you ask the question 'will it invalidate my warranty' then keep it stock as you will not be disappointed
Also if you can't afford or don't want to pay if it goes wrong don't play