I've actually given a lot of thought to why I can't find anyone replacing the 4th gen VVTi 1GR with the 5th gen dual VVTi 1GR. In theory, it's the same engine and, in theory, should bolt up nicely. It would require a new ECU, maybe new exhaust headers, and maybe new secondary bolt-ons (alternator, starter, etc...) but you'd get more power than the v8, torque between the v6 and v8, and better mpg than either.

Now, after reading this, it definitely wouldn't be that easy. Replace EVERYTHING, and hope the bolt patterns on the engine mounts and the torque converter and transmission mounting patterns align, which they probably wouldn't...