My 2 cents on this as I toyed with upgrading mine as well and did a hell of a lot of reading up on this. (I probably won't do mine at this stage as my mileage is very high and figure if it ain't broke... If my turbo shows signs of going I'll look at changing but for now to be honest I'm actually pretty happy with how it is. My requirements are modest and it's a nice all-round balance between having a bit of poke and economy as it's a family car and daily driver, and I mean daily - it's rarely not on the road.)
Anyway, what I'd say - don't fully write off the tf035 that's in it. It's the baby of the bunch but it also spools the earliest of them all (around 1800rpm) so yeah while it won't keep up to higher speeds it's well able to haul itself off the line. TD04 supposedly won't get much of an increase - probably looking at around 240-250hp max realistically but it's slightly slower to spool up (starts around 2000rpm). Supposedly you can squeeze 220/230hp out of the tf035 but I presume that'll be seriously pushing it and I'd guess you'd need at least a decat with that.
I left mine completely standard apart from pro-drive backbox (presume that's only affecting the sound) and Aonghus Power got 202bhp out of mine (I can't remember the torque figues but they were decent - typically his printer was dead that day and he was meant to email me on the figures but never did). At the time he said had he mapped with NF added he'd have gotten more out of it. He also commented on the size of the intercooler, they're tiny. Bigger one would help squeeze another bit out of it as detonation risk is lower with the cooler air - wrx intercooler is about 25% bigger, STI one obviously better again but impossible to find. (I have a newage wrx one (cheers Clodex) to go in at some stage for mine)
Anyway point being I was back and forth on the idea of swopping out for TD04 as well and came to the conclusion that it wouldn't make enough of a difference to make it worthwhile. VF34 is supposedly the best all-rounder for the forester but they aint cheap. VF28 is another cheaper option that people have gone with but they are a late spooler at 3500rpm but will keep puffing right to redline. Of course the next problem you're gonna have with those is you're limited by your fuel pump and injectors - so there's more money. And of course now that you have the fast-ey bits better you'll need to look at your slow-ey bits cause the brakes on the fozzy..eh could be a bit better.
Good thread here on the tf035 :
www.subaruforester.org
And just in case yer tempted :
VF34 :
https://www.4turbo.pl/turbocharger-ihi-vf34-impreza-sti,3,49763,23082
VF28 :
http://jdmperformanceparts.co.uk/subaru-impreza-wrx-sti-vf28-turbo-charger-very-low-milage.html
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