I have the same thoughts. As it just a WRX then maybe not. STI's and specials are going to be the ones people look for. Mine is WRX 2.5 Hawkeye with 45K miles like you 2 owners. Mine I use as a Daily Driver so will depreciate anyway. I think it will be taxed till it is worth nothing. Cities will charge us the earth to enter which will make them less valuable. Oxford is changing to £10 a day soon for example. The rest will follow and towns.
As yours is highly modified I am not sure but mine is just going to plummet in price I believe sooner or later. Mine being 2007 model is already double your car tax nearly. You really need one in your life to be prepared to pay that these days and that just increases every year. Also 97 ron or higher is going to go up and up and be harder to find.
My problem is for 10K (same sort of money I spent on the WRX with some extra) what can I replace it with that will give me that fun/smile factor. Mine is mapped and the mid range now is addictive. Modern cars frighten me, so many electronics and no much skill for this in your normal repair shops (and Subaru dealerships stink) and there is only so much I can do myself in the street outside my house (if I can park it there, that is another story).
Selling now or soon is probably a good idea for me but I cannot do it.
I will miss it when the day finally comes.