Opinion on value.

Had VWR springs on my old MK6 GTI and received a sale quote from Carmax and because of the springs they dropped the appraisal by $1k and said if i returned back to stock the appraisal would go back up.. sold car PP to an enthusiastic fellow

Mods will not value much until you find the right buyer. Your best option would be to turn it back to stock and sell the mods separately. Yes it involves a lot of work, but it might be worth it. I went through the same thing recently, although i did not have an extensive mod list like that of yours.

In general, even the options from the factory does not add a lot of value during resale. My GTI had almost all options from the factory and when i had listed it for sale, people would just low ball comparing it to a regular GTI.