Wow, this discussion again? There are tons of threads from April or March onwards about this stuff, we talked about it at great length.

First off, I and several friends with 300+ whp wrx's running 5 speeds have had no issues with the 5 speed, even after 200k miles. Remove 1500 lbs and make it 2wd and the 5 speed will probably have little issues because the fight is putting all the power to the ground.

Just don't drive like an a$$-hat, OEM transmissions on the mighty GT-R broke left and right too. Why? Nissan programmed the launch control to drop the hammer at 4500 rpm and had the foresight to void the warranty on the transmission if you used the launch control! How did Nissan fix their launch control problem? They changed the launch control rev-limit to 3000-3500 rpm, suddenly the transmissions stopped blowing up and somehow the 0-60 time got even faster.

I use a free ECU 'launch control' patch on my WRX and have the launching rev-limit set to 3500 rpm's. That's where the turbos just begin to spool and it's not so fast that my transmission destroys itself, I just flatfoot the throttle from a standstill and quickly engage the clutch (note, I didn't say "drop the clutch) and boom, everyone is behind me and my gears still have teeth to boot.

2nd, the STI's gearing is short, very short. On a 1800lb car, you'll be hurting your performance by having such short gear ratios, I mean 2 shifts to 60 when 60 will be coming in 4 seconds or less is just dumb. Also, achieving full boost is difficult in lower gears because the engine simply has too much mechanical advantage to build full boost easily, the engine will build revs faster than the turbo can build boost. Shed 1500 lbs, gearing as short as the 6 speed's will just make that even worse.

3rd, the STI transmission has a very complicated, very fancy and expensive center differential. It's a DCCD or Driver Controlled Center Differential. It's the bees knees for an AWD subie but in an 818 you'd have to weld it shut like a drift car or ditch it for a solid dummy piece in it's place which doesn't exist (while there are pre-made options available for the 5mt). That's like a $1k part that you have to destroy or remove.

If you buy an STI donor for an 818, you're literally going to be better off parting out the 6mt, it's a very popular upgrade for other Subies and buy a 5 speed to either drop right in or use the extra cash to upgrade it with PPG or RA gears and one of several viable and inexpensive front LSD upgrades.

If you're looking to put more than 400 whp in your 818, yeah, you might want a stronger box than a stock 5 speed but honestly the 6 speed isn't the answer to that in an 818. Fortunately, there are lots of strong aftermarket 5mt gear-sets available.

Save the 6 speeds for the AWD guys, they actually will benefit from them.