I was using -10's, -5's before that.
Packed, hot and hard to work on, especially when you have to replace them.
A few years back I did a comparison between a T04Z and twin -5's and the twins won hands down on response, from that day on I've been a twin guy.
However having said that, these new twin scroll efr's offer a much better result as a single, where response is no longer an issue with a bigger single. I'd love to whack an 8374 on my motor now to do the comparison again as I think it'd probably be close enough in response to now choose a single over twins.
Given what I went through, if I was to do it again I'd probably go a single 8374, just for the cost saving in fabrication and ease of maintenance.