So would it be better to just run glasspacks or something like that? Since they are technically straight-through mufflers.

Those are tricky also.  If you look inside you will see the holes and inner chambers in them.  The exhaust pulse actually gets trapped inside (even though its for a split second) and hits the next pulse causing turbulance or backpressure.  If you will notice you here that sputtering on some of the imports with the cheap mufflers or glass packs thats the exhaust roll over actually causing lost off idle torque and decreased throttle response.  The saying you get what you pay for is absolutely true.  Thats the reason you will see the high prices on tanabe, greddy, hks, magnaflow etc.... They really do make good products.  I do believe though that some of the new glass packs have a better design.  Now on the top end the mufflers flow good because its a constant pulse.  Most of this isn't as bad with auto's but if you got a stick its very apparent.