So the cartridge is currently seeing 225pF. Do we have Ross Moloney to thank for that info ?
When building my loading box I looked at recommended capacitances for a selection of cartridges up to £500.
200pF or less - all AT, Mofi, Clearaudio, Goldring 2000 series.
Ortofon Red 150-300pF, Ortofon OM5 derivitives and Concorde 200-500pF.
No recommendation - Sumiko, Project, Grado, Nagaoka.
My old Shure V15/III 400-500pF.
I'd like to think that no recommendation meant the cartridge wasn't fussy, but the graphs in the link I previously provided ( MP-11 test) suggest this is wishful thinking.
Are you suggesting that you could replace the existing Project cable with a 3m cable ?
If so here's a leftfield suggestion for £7.99 (was £74.99) and it's from a site sponsor
Fisual Super Pearl Silver Reference Component Video Cable 3m - Component Video Cables - AV Online - UK Home Cinema and Hifi Specialists
I bought some 2m versions for £4.99 to use as interconnects because decent RCA plugs alone would have cost more. I've measured the capacitance as 60pF/m. It's very well made and fully shielded.
So replacing the existing project cable would mean the cartridge would see 280pF (3x60 + 100) instead of 225pF, which is unlikely to have much of an effect. I'm not necessarily suggesting this as a permanent solution as sadly it's not possible to cut the 3 cables down to a pair due to the heavy welded-on black strapping you can see in the picture.
However it would give you the opportunity to see if there were interference issues and how critical the capacitance issue is. Then you could decide whether to go for just a long cable or the expense of a phono stage.