I think this debate must go through all WRX owners... I know I've been debating about it ever since Subaru sent me a notice in the mail for a guaranteed trade-in amount for my 03 wrx for an 05 model. All I could think about for days was if it was really worth it and if I could afford it.... Honestly, I could afford it but I just couldn't justify it. A did say I could afford it but not comfortably.
I haven't modded my car to the extent you have. I'm at cobb stg2 with some exterior mods but I just couldn't justify trading it in. I also figured in that my car will probably be paid off in about a year, which is great because that's more money in the pocket... and more money to mod. If I traded in I'd have car payments for at least another 3 years.
It's a tough choice but if you have money to spare and are willing to take a loss for a trade in... go for it !! I'm stickin w/ the wrx