I agree, the 19T TD04 is closer to a stock TD04 than a VF39. Basically the upgrade will net you close to 20% more air flow, giving you those HP gains with spool very similar to a stock TD04. If you like the feel of a stock TD04, then the upgrade is a higher powered version of it. Personally, I've run a VF35 and an 18g, and have gone back to a TD04 simply for the quickness of it and the powerband centered around 3K, which is pretty much what you run at driving around under normal gearing. However, it's not as fun on the freeway ramps as samo22 had said
It is not as fast as my 18g when revving through the gears, but has more passing power without the need to downshift at normal driving RPMs.
Now, I like power as much as the next guy... and honestly I'd love to run a big turbo. However, being stuck with a 5 speed, I've decided to stick with a turbo that has power in a more practical range. If I ever get a good chunk of money saved up, I'd love to do a 6 speed swap or a built up 5 speed, and then run a twin turbo setup similar to Borg Warner's RS2 setup... which they call "regulated 2 stage turbocharging". It's pretty much a hybrid between a staged and sequential turbo charger setup. It behaves like a sequential, but is built similar to staged. It would have to be all custom for a subaru, and thus lots of $$$, but could potentially give you full boost from 2K to redline with the right pairing of turbos... which I think is pure evil
... in a good way.
As far as my downpipe, it's an Ultimate Racing one with a metal substrate cat, rated efficient to 700 whp. Given marketing hype, assuming it's only good for half that, I shouldn't see a restriction issue with it. My tuner did mention it would be better if the cat was located farther away from the turbo through. It's my only cat, so I kind of want to keep it. I'm also running an EWG that by-passes the cat, so that should be helping a bit too. Many people feel that the stock inlet isn't the restriction, and I'm sure it's not "that" restrictive. However, if you're running your turbo at its limit, every bit of pressure against it reduces its ability to produce boost. A big turbo has plenty of ability to work against it, a small turbo does not. So I'm still thinking my inlet hose may be limiting me, especially since Steve is seeing better top-end results running an upgraded inlet hose. I could be wrong as there's so many things that play together, and the engines are different, so I guess I'll find out when I do my inlet upgrade.