RP and 100-400 EF (not version II) and 2X Canon converter

jboyer wrote:

I fitted this combination on the RP while questioning the idea of trading this for a RF 800mm.
I believe the 100-400 is a better lens. But for birds, 400 is somewhat short.

Suggestions and opinions?

Thanks.

My news isn't exactly good...
The EF 100-400mmL Mk 1 is an older, 22 year old, pre-digital lens with a slower AF module that was designed for film cameras and not for digital cameras. It will WORK on a modern digital camera but Canon do NOT recommend using it on an R-Series camera due to the age difference. The performance is therefore unreliable and slow compared to the Mk II lens and Mk III Extenders.  The EF 100-400mmL Mk 1 was announced in September 1998.  This is something of a prehistoric lens when used on an R camera.  I've had arguments with other photographers who claim their Mk1 lens and Mk II Extenders work just fine on their R cameras... and then later, I tend to see the very same people complaining on the Facebook R groups about optical quality and under-performance.
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The EF 2x II Extender is likewise a pre-digital Extender that has a slower, older microprocessor in it.  This Mk II Extender is also optically inferior to the Mk III Extender and again, Canon do not recommend the use of it with modern digital cameras from the 6D Mk II onward.  And the 6D II can only use the EF 1.4x III Extender for AF via the Viewfinder.
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The newer EF 100-400mmL II was released in 2014 and has improved AF designed to work with modern digital cameras.  The same applies to the EF Mk III Extenders, both of which were redesigned completely to improve optical performance and have a far stronger redesign for structural integrity and new optical coatings.  They also have new microprocessors with faster AF speeds and greater accuracy when used with the EF 100-400mmL II lens and similar Mk II white-series L-lenses.
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If you don't believe me, contact Canon directly and tell them what you are using.   The EF 100-400mmL II lens performs quite closely to the RF 100-500mmL lens in terms of AF speed and accuracy... even with extenders... but this performance is more notable on the R6 and R5 cameras which have an improved AF module and a more advanced Eye-AF detect system compared to the RP and even the R.  Due to the higher megapixel count on the R5, these lenses actually perform a little better on the R6 (as noted by other members here).
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Solution:  Trade in your Mk 1 lens and Mk II Extenders and buy the EF 100-400mmL II lens and the EF 1.4x III and/or EF 2x III Extenders.  You could probably get a good price for new gear... although second hand equipment is an option but ONLY if you can be assured it is optically and mechanically sound.  Unfortunately there will always be a performance issue using the older lens gear (especially from 1998) on a recent model Digital Camera like the R-series.