egt cht

I'd like to add an electronic six probe egt/cht to my bonanza. The
three usual candidates with similar products are
Electronics-international, JPI and Insight. Some have better features
than others but all do what I need. I don't need to save data for
future reference. Dynon and MGL electronics both make LCD monitors for
sport/experimental aircraft that accept standard probes. Neither MGL
nor Dynon have any FAA approvals. Dynon says that you could get a 337
to install one of their products. The basic MGL TC3 would be fine for
my purposes. It's $265.00 plus the probes plus installation. The
cheapest approved monitor is around $2000 with probes plus installation.

Has anyone gotten a 337 for the installation of any of these products?
Would it help if I left my existing single probe EGT and CHT and
their associated gauges?

I've read that these gauges can be installed in sport aircraft and that
these aircraft can be used for flight training. Maybe I read that
incorrectly but if that is the case, how could the these products be
used in airplanes used for hire but not for a part 91 airplane like most
of us fly.
Thanks
Dave