The 400r weighs about 200 lb less than a pair of 2.5 200's and will be faster as well. But no, I wouldn't recommend one on a hydrostream or Allison. But what's coming later this week? We will find out soon enough.
Anyway, yes the 400r is expensive. Or is it? In 1991, the 260 msrp was over $16k. That equates to over $35k in 2018 dollars. The 400r starts at $31k (neglecting that fact you can buy them for $26k), so for 10% less inflation adjusted dollars, you get 50% more power, worlds better reliability, fuel economy, features, and user friendliness. Don't get me wrong...I thought people were crazy spending $16k on an engine back in 1991.