So far, the M2200 (got it for $100 a while ago) proved to be a substantial improvement over K2100 and it seems like M4800 (and probably M6800 for that matter) can be made quite useful even further - the SSD can be expanded to 2+8+8TB. Since GPU is used in both work and entertainment this is a single incredible performance upgrade. We don't see this to be possible in notebooks world that often.
Now I'm curious about T1000, T2000, A1000 and A2000. Although, they are hard to find in MXM A form (please share if you know where they can be found and if they are compatible). There are some ridiculous $$ offers targeted at embedded applications. I haven't seen any of them with X-bracket depicted, probably worth looking into.
I will get Honeywell PTM7950 for CPU and GPU, and maybe new GPU/SSDs. I already got new back plate, palm rest, bezels and it looks pretty clean now.
Insofar, the disappointing limitation is... DP1.2, since Haswell DP is limited by 540MHz clock and 4k&60Hz&8bits is 533MHz. I have a S95B QDOLED with 4k 144Hz 444 10bit and am limited by 4&60&8bit, 1440p60 and 1080p144. 144 is silky smooth. Surely, M2200 will not run a lot of titles well with 4k & 120/144, but even newer cards will be still limited by DP 1.2.