Does anybody know if King KX155 nav/com can be converted to 8.33khz?
No it can not. You would be looking into replacing it. A Garmin GNC-255A would be suiteable, or your might want to upgrade to a COM/NAV/GPS navigator.
Bendix/King alternatives are quite expensive and 28 volt only.
Out of interest, was there ever a KX165/8.33?
The KX165A/8.33 is expensive and very hard to get due to heavy demand from Europe.
The KX165A is tray and pin compartible with the KX155
With the KX155A only. The KX155 has a different pin layout.
Out of interest, was there ever a KX165/8.33?
No, just the 28 volt KX-165A. And a KY-196B (COM only) also 28 Volt. Both are very expensive units. The KX-165A lists for 6180 USD and the KY-196B for a massive 7410 USD. A GNC-255A lists for 4495 USD, so including installation would still be in favour of the Bendix/King alternatives.
The KX165A is tray and pin compartible with the KX155
This information is incorrect, the KX-165A requires longer molex connectors then the KX-155.
I am reliably informed that somebody has found a way to do this with a KX165 
I don’t have the details, but presumably it would be legal only on a homebuilt aircraft
I am wondering how they release that modification ?
Even with an official exchange unit you need an EASA approval because it is an equipment change (other Partnumber)
It would require modification on the receiver, transmitter, audio, oscillators, display control, I doubt this true, as it is quite some work, would be more sensible to start from scratch. I am also quite sure the unit is not allowed on homebuild aircraft, as it will violate the excisting FCC approval for the KX165
It does require a new front panel.
The mod was developed by a private person 
Yes you are probably right about the FCC approval.
However I am pretty sure that a perfectly usable 8.33 operation can be achieved simply by modding the DPLL to step in 8.33 steps. If that was not true, any use of existing non-8.33 radios would wipe out all the intermediate 8.33 channels. Yet, we have lots of non-8.33 radios and will probably, frankly, have them for ever, existing within the GA community, in areas where radio checks are not needed (some homebuilts, etc). And 8.33 does work right now, with non-8.33 radios operating on the 25kHz channels right next to the 8.33 channels.
Detection is very unlikely since so much GA flies with radios so crappy they barely work.
