The TKO is built to the exact same technical spec as the T56. You can't build a T56 any stronger than you can build a TKO. The only reason people are getting away with putting 800ftlb of torque on these Viper Spec T56s is because they aren't launching the car hard on slicks. The Magnum T56 is built to a higher technical spec, can handle more torque, but can't be shifted as quickly due to the engagement setup on it. From what I've learned over the years, pouring money into a T5 is ****ing upstream. The size of the case is a huge limiting factor. All the bandaids in the world won't keep you from cracking the case. The gears are too small. The only way to overcome the size of the gears is to put straight cut gears in, which cause the transmission to make a horrible whining noise.
The 3550 series was not originally designed for drag racing, and that is why it has trouble shifting quickly at high rpm. There are ways to modify it to shift faster without going all the way to a face plate setup. Mine shifts pretty quick. As quick as a T5 at least. When buying a used one, the main item to look out for is gears. One gear set in a 3550 will run you $300. The rest of the parts cost more than T5 parts, but no where near as much as T56 parts. Put the transmission in each gear and rotate the input shaft. If it is notchy at any point, it's probably a cracked tooth. The 3550 series is very easy to take apart and put back together with simple hand tools. It is very needle bearing happy inside. So long as you don't mind spending a little extra time stacking needle bearings, it's real easy to deal with.
As to whether or not it's a good deal? The first thing we need to clarify is your location. Are you really located in a Province of China?
Kurt