If you compare the picture of the TX in the shop, and the picture of the SX model (removed from the shop, but still available on other sites) you will see that even the silkscreen print on the top board side is the same, watch left side with the "MEDIATOR 1200 SX" line - a "TX" would look like different.
Even the component placement is the same...
So IMHO it is a nice marketing gag: you fix some parts of your programmable logic code for the CPLDs, make some speedups, announce this as a new product and charge 20EUR more for the physically same boards which you have (even after several "free shipment" events) plenty on stock.
Other companies would call this a firmware upgrade and offer it as user service, not as new developments :-)
About the DMA discussion: not again. At least on A3000/A4000 Mediators it would be possible from the bus interface, but not with just some old MACH chips. Real Zorro III DMA needs more logic, especially when working around Buster bugs.
Michael