I think mid 13s is pretty dang good myself. It is a one off so it isn't perfect. The SOHC wasn't exactly built with twin turbos in mind. They're probably keeping that in mind for safety sake. BTW, twin turbo supras, 3000 gts, and 300zxs are all slower than that with their factory twins.
And sometimes it doesn't matter how many blowers you're running. Why do you think all the supra guys try to convert to singles?
On the SOHC, if you look at the timeslip page on RPS, you'll see that no supercharged truck has reached 13s. Orion hit 13s but he needed nitrous(and it still wasn't as fast as the TT4.0), off the juice I believe he ran low, low 14s. The explorer express guinea pic Nick Blakely is still in the 13s as well.