Have no experience using these but wanted to try them. When researching the K1 and KF related ball (MIP # 14275) and gear diff (MIP # 14280) versions, I found postings describing increased wear compared to the kit units, at the diff output and the axles. There are also postings describing them as an "Excellent upgrade". For example the rctech.net K2 wiki:
http://www.rctech.net/forum/electric...l#post15015596
Both of these units (MIP # 14275 and 14280) use aluminum diff outputs which should be softer than the steel SV2 (MIP # 14100) versions. The SV2 (MIP # 14100) should be compatible with the MIP roller pucks as well as the plastic orange versions.
A potential feature of this system is that the dog bone sections appear to be interchangeable with other MIP pucks dog bones. For example, exchange a T4 related puck dog bone with a KF-series puck dog bone to build a stadium truck. Given the possibility of wear, it would be preferable to have versions of 14275 and 14280 with steel diff outputs.
Based on the Schumacher exploded diagrams, the SV2 related MIP 14100 and KF/K1 series related MIP 14280 kits could be compatible with the new KC and KD, however the MIP puck gear diff output (MIP 14281) is aluminum and the SV2 related MIP ball diff output (MIP 14101 & 14102) is probably shorter than the KC ball diff outputs (compare U3750 & U3751 to U4005 & U4006).