Phono preamp worth it with MM cartridges?

When to take the step to MC? I would say you would get the most bang for buck by staying with MM. Get the 2M black (or bronze which you can upgrade to black for little money in the future) and partner it with the schiit mani. The mani is amazing sound for the price. That combo will sound so much better on your Rega turntable than your current set up. I loved that combination on a similar priced project deck a few years ago. I only moved to MC when I could afford to upgrade to rega P8, apheta 3 and rega aria. Aria is great but haven't tried it on MM. Am keeping the mani if I ever get the chance to have a second deck with MM. 

Yeah, all you need for LOMC is sill a decent MM phono stage and external SUT to match cartridge impedance. I’ve been using this pair of SUTs and it can cover all types of LOMC (from low to high impedance).

MM stage and SUT is a classic combo.

A great MM phono stage is a must have, in my opinion great MM stage should not cost more than $1200 (many available withing this price tag).

If you have a friendly dealer take the $2k and your P3, and exchange for a P6 with the Ania MC + Sutherland KC Vibe phonostage. THAT will be a very noticeable upgrade to your record collection.

@chakster, "todays MM cartridges are 95% junk." 

That is quite some generalization chakster. Do you have any additional references on that? Granted, the cheapest of the cheap cartridges are moving magnet cartridges but I do not think any of us take a $15.00 cartridge serious. 
Since you think MM cartridges are superior to MC cartridges does that make 99% of them junk? 

I share the general disrespect for the Exact, but defend the MM pre in the Elex-R.  What you want is a better cartridge, perhaps a Benz Ace HO?  Boron cantilever line contact nude stone, for under a grand. Another good one is a Hana SH.  And order a set of AS shims.