Realistically, how many kilometers do you expect to rack up on that engine after the rebuild? 50,000? 100,000? People have regularly gone further on engines without balance shafts without crankshaft or bearing failure.

So then, how about the vibration? Well, the range that generally gets worse (from what I hear) is the 2-3000RPM range, and possibly beyond that a bit. That's right around where you'll sit at on the highway (with your Evo3 fifth in). I know you're the type of person who enjoys a quiet, vibration-free cabin. It may be worth it for you in that sense.