[KobaltBlau]

if you boil it down to $/hp just about any peformance upgrade on a naturally aspirated porsche engine is hard to stomach.

"There is no replacement for displacement" overall torque levels (the area under the torque curve) is tied more closely to displacement than anything else. This means you gain power down low, up high, and everwhere in between. whether the $3k is worth it to you is another matter. Note that if you have Alusil cylinders, you can't "just re ring it and call it good" without some careful attention, some would say they are not reusable at all. if that is the case, you might as well go to a 3.4. Another possibility is to have your cylinders overbored to 98mm and replated, even if they are Alusil. then you can buy JE pistons and the whole setup costs less than the mahle stuff. Some would question the JEs for an engine intended to go 100k+ miles before another rebuild, depends on your goals. this makes a 3.4 as well.

I wouldn't do a displacement upgrade without doing cams, just because the cams are a good upgrade and not too expensive. 993SS cams might be too radical (talk to camgrinder) but 964 cams are very good. I believe 964 and 20/21 cams are extremely similar. 964 cams don't require new springs, retainers, etc, and I probably would leave those parts stock. of course you do have to get the rockers resurfaced to match the new cams. If you have bad valve guides (is this why you're rebuilding?) you may need to replace valves, but the retainers, springs, etc don't strictly need to be done.

As to the custom gears, this is a good way to look at things (thinking outside the horsepower box). But, you have a problem, because you have a G50 custom gears are pretty expensive. Even with a 915, though, you have to look at your usage. The gears are carefully spaced in both transmissions, so it's hard not to replace most of them. For a street car, you probably still want a high fifth if you do much highway, and a relatively low 1st compared to a race car. This doesn't give you much leeway. Conversely, with a race car gearing can make all the difference, but then you have more flexibility in terms of spacing things closer together without caring about around-town and highway driveability.