My overall point is that GHz-speed decoding in a flexible manner seems... difficult... to say the least.
Your discussion point of "here's a VHDL block" seems to understand the general issue. You need a non-trivial amount of FPGA-magic (LUTs) to implement logic and routing at those speeds. SATA has some kind of error-correction code if I remember correctly... so its not exactly easy to parse those messages.