I wouldn't say they all suck...In your case the center/offset 2 1/2" are pretty restrictive mainly because of the obvious neck-down and the case width being too narrow to allow the over/under flow around the center tube. But the Larger 3" ( ex #17769) Dmax ST of the same design has a taller and wider case and actually has eqaul to or slightly MORE cross sectional/flow area around the top and bottom [roughly 2X(3 1/2 x 1 1/4) or 8.75 sq in vs 7.068 sq in for a 3" pipe) of the center tube than the cross section of the pipe ID itself so the weak point you make is not a serious flow restriction issue. What you have is a compromise of overall case size.
I like the large case 3" muffs but not the necked down small case 2 1/2" like your example, there are some very good flowing Super Turbos that give up comparatively little performance vs the ultra-flow compared the considerably lower DB levels. Whwen properly sized the flow of the ST is more limited by the reverse flow than by the internal volume.
http://www.dynomax.com/mufflers.php?muffler=superturboI like the ultra-flows but on my 700+hp 12:1 roller motor with a ton of overlap....so for my motors they're simply just too Loud for the area I live. When my car goes by people look for the right reasons...which is worth giving up (maybe)a tenth to me any day.
Arguably a better design (in terms of DB reduction) was the Edelbrock stainless muffs that have the UF straight through but the pipe splits into 2 and rejoins inside the case...like 2 y's joined at the top....this gives you more perforated area to reduce the db's. I have a set of the 14" on the 414 Rocky and they're not too loud, seem to be better made and and are noticably quieter than the Ultra-flows.
