To think that, even two, independent GPS installed with zero integrity monitoring and uncontrolled databases coupled to ad-hoc processes is a sign of good airmanship
No GPSs are "installed" without intergrity monitoring. Every TSO'd panel mount has RAIM and most have RAIM with Fault Detection and Exclusion. Every TSO'd panel mount I know of has a database supplied by Jepp or by Garmin from Jepp. Both these hold Type 2 LoAs showing the same conformity to Do200A/ED76 as airline database suppliers.
or that it can, just like that, approach the level of performance achieved in the "professional world" is, to my mind, a questionable proposition!
GA GPS installations frequently fly to a
more demanding level of performance simply because the US now has more WAAS approaches than ILS - this is more demanding than any enroute/terminal RNP application I can think of. In Europe, the most demanding is P-RNAV (RNP1) which, ironically, the GA fleet with its fairly homogenous GNS430/530 kit can conform to more easily than many older airliners with legacy FMS.
Sorry VOD80, what ever your point about GPS is, trying to make out that GA IFR GPS is inadequate is not helping your cause.