RPM limit for a built VG just depends on where you want to limit it. For instance, there was a GTP race car powered with the VG30E. Fully built they were running it to 8,500 rpm. I personally wouldn't aim much higher than 8,000 for the DETT and 7,000 for the 30/33E. When you design an engine to have power in the higher ranges like that you generally lose power in the lower ranges making them a bit less street friendly.
Stock block I believe they say the DETT is good to 500-600 and there are VG30ET's out there making 500 whp as well.
The pistons will be the first to go in a high power situation so those need to be replaced with some forged pieces. Rods would be good to replace with the pistons just for the added benefits of them being lighter than stock, as well as stronger. There are plenty of rods out there for the DETT and they can also be used in the VG30/33E. Only thing with the SOHC VG is that the piston needs to be designed to work with the DETT wrist pin, which is simple for the manufacturers to do.
As far as weight, the SOHC VG's will definitely be the lighter of the two.
I'm actually doing the SOHC VG in my Z32 because I absolutely love those engines plus it has the added benefit of being cheaper and much easier to work on because it is so compact:
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http://www.machzracing.com/300...1.jpg- http://www.machzracing.com/300...2.jpg- http://www.machzracing.com/300...3.jpg- http://www.machzracing.com/300...4.jpg- http://www.machzracing.com/300...5.jpg- http://www.machzracing.com/300...6.jpg- http://www.machzracing.com/300...7.jpg- http://www.machzracing.com/300...8.jpg
Granted there's nothing mounted at the moment, the big thing is how much space are between the towers and the heads, quite a bit more space than the DETT.
300ZX VG30DETT:-
http://www.machzracing.com/300ZXTT/Engine/TTZ2.jpg- http://www.machzracing.com/300ZXTT/Engine/TTZ3.jpg- http://www.machzracing.com/300ZXTT/Engine/TTZ4.jpg- http://www.machzracing.com/300ZXTT/Engine/TTZ5.jpg- http://www.machzracing.com/300ZXTT/Engine/TTZ7.jpg
Either engine will definitely be fun. The DETT is a little more, I guess you can say refined, than the VG30/33 and can most definitely make some insane power but I'm a sucker for the VG30/33 because it can also make a lot of power while being more compact, lighter, and cheaper. I actually bought 3 VG30E(T)'s recently for $225.