In regards to the Hacman type device, I have been using one on a Jab 6 for 5 years and over 500hrs experience now. They do work well, but must have full EGT, CHT and fuel flow monitoring to get the best out of the system, and to avoid damage. The valve itself is quite sensitive and only small incremental openings are required. When confidently used, benefits are enormous. I can easily run LOP, fuel flows dropping down from 22LPH or so to only 15LPH, with correspondingly lower CHT's, of course an increase in EGT's.
The caveat here is that the engine must already have close EGT's when set up in the cruise mode, a well matched prop etc.
This system is only of benefit above 5000ft or so, and a long cross country flight, other wise the bing does a pretty reasonable job in the lower altitudes.