Considering that the driver comes with the controller, I think this is a rather silly point, no offence. Why bother splitting this hair? In all practicality you have to buy a controller that supports TCQ.
Besides SR, notes that the differences between the S150 TX4 and the TX4200 lie only in the firmware realms. This implies to me that NCQ can be enabled in firmware/drivers as well.
As for you performance concerns I think they are unwarranted.
1. ATA TCQ uses DMA to return the answered requests, just like NCQ, the CPU utilization isn't going to be different.
2. If you're taking issue with managing the registers for the commands and tags, or the overhead of the actual tagging I don't think there is enough information available to distinguish between the ATA TCQ and NCQ. Is your concern that ATA TCQ implementations use main memory space for the registers to store the commands instead of registers on the controller? I think it is likely that the implementation of NCQ on the TX4200 is just as 'software' an implementation of TCQ as ATA TCQ is on the same controller.
I still disagree! Again, TCQ is a matter of drives supporting it and a TCQ capable driver. NCQ requires a NCQ capable drive (basically a TCQ capable drive), a NCQ capable controller, and a NCQ capable driver. TCQ does not use DMA to inform the host CPU about a served request, its a task of the host CPU (polling). NCQ is much smarter, since you just load a bunch of commands into the controller (to be precise: the host memory, a command chain) and wait for completion (indicated by an interrupt). You can entirely offload the processing of the command chain, in difference to TCQ where the host CPU has to find out, which request can be served, then arm the DMA engine of the controller.
just my 2 cents...