I used a Discovery Plus4 phono cable with my TT for a few years. I really liked it. It is extremely musical. It's main emphasis is mid range and has a great HF charecter. The RB300 wire upgrade from Discovery uses Plus4. HiFi+ did a great review of the Discovery-vs-Incognito wire. They choose the Discovery Plus4.
Having said that I can say that now using the Discovery Essence phono cable has shown some of the areas lacking in the Plus 4, namely bass. It does not go as deep as the Essence or have the overall bass impact. The Essence HF also exceeds the Plus 4's a bit. BUT, the Plus 4 has a magic that the Essence can't touch. The Essence can't be put into the RB300 so that is not an issue.
Now I recently put together a 2nd sys and have a RB300 with the Michell weights and the Incognito wiring. I have yet to set it up so I can't really say how it might compare.
Some people really think the PT6 may be better than the Rega. I asked just this same question here and on the "VA" site. I think if I was in your shoes I would upgrade my cart. and save for a better arm than the RB300, say a 10" VPI or SME or a OriginLive.
I'd go for a Pete Riggle VTAF over the VPI unit. VPI made two different units for VTA. I have read some +/- about them both. I will probably pick up a Peter Riggle VTAF soon.
He makes a nice heavy weight as well.