tonearm for Gerrard or Lenco idler wheel turntable

I am using a Lenco Idler wheel TT with a Jean natais modified plinth.

I wanted to know from Gerrard or Lenco tt owners or fans whether upgrading from a rega rb700 to a better arm will reap substantial benefits.

I have a strong phono preamp the Dartzeel preamp and amplifier as well

I am using the recently reviewed and very well liked Ortofon A-90 cartridge.

I was thinking about a phantom b-44 II but am a bit wary about a unipivot as opposed to a more traditional tonearm.

Anybody try a Reed Tonearm versus a Graham on a Gerrard or Lenco TT and find one a better match than the other?
I don't think you need to worry much about "matching" a tonearm to a turntable. The question is whether the turntable is "good enough" to merit a top line tonearm. I am a previous owner of a Lenco in Nantais plinth and the current owner of a Lenco in a slate plinth with the PTP top plate from Peter Reinders. The short answer to your question, IMO, is yes, a Lenco in a well-built effective plinth is certainly good enough to benefit from the addition of an expensive tonearm. Which tonearm you choose has more to do with your choice of phono cartridge than with your choice of turntable, IMO.
i think Lewn has made some good points.I have ( and will have ) most of bit parts/ gears you mentioned in my room soon but they are all scattered on different setups. I don't think I can give you clear answer to your questions with a direct comparison at the moment but I think A90 will match well with the Phantom I/II judging from the experience of some users here. Don't worry about Phantom being an unipivot and deter you as it does behave like one with the Magneglide.
I have heard Jean's latest Uber all-out effort that was commissioned by a prominent reviewer. It sports a Phantom 2 and sounded wonderful.