Phono preamp worth it with MM cartridges?

Roger, Listen to the one you have. If it sounds OK to you then don't spend any money. Once you need more gain then you will be forced into buying a new Phone amp. Spend the money on getting a better cartridge.
There are some fine MM cartridges available. 
What we think sounds good is not the point. It is what you think. 

There are some killer MM cartridges, but not your Rega, you'd better change a cartridge first.

Some MM or MI phono cartridges are more expensive than MC.

A phono stage is important for all types of cartridges.
There are many inexpensive MM/MC phono stages on the market, the one I really like is JLTi mk4 (now mk5 available from the manufacturer).

Different loading for MM is also a very nice option, so the loading is NOT only for MC.
Loading for MM must be from 30 to 100k Ohm. A phono stage with RCA-Plug resistors for MM is nice to have, but internal resitors in the phono stage must be very high in this situation (500k Ohm) to load it down with parallel RCA-Plug resistors .

@chakster thanks, what to your ears would be a better sounding MM? My local dealer offered me the 2M Black which I'm open to as a long time fan of Ortofon but he thought it comparable to the Exact. I guess there's no point buying a better cartridge than the deck/arm I have can do justice to?

I really wish the dealer had talked me into a getting P6 to begin with. The other option at the back of my mind is to do neither and move the Exact to a new P8 and sell the upgraded P3.

Seriously....not just bashing...but get rid of the Exact cart...literally, every other cart I’ve had (including less expensive ones) has sounded better (at least to these ears)....my opinion is that it may be handicapping everything else.

Makes me wonder how many Rega Exact purchasers have been unnecessarily turned off or left unimpressed by vinyl.

For sure though...this is just my experience/opinion....So, grain of salt...

wqgq_641, your arm is a light intermediate mass arm. The 2M black should work well. The Clearaudio Charisma would work perfectly. IMHO it is one of if not the best MM cartridge available today. It is also priced that way. The Clearaudio Maestro V2 is a bit less expensive and also excellent. There are the Grado cartridges. The Ref 3 is a standout. The Master 3 is about the same price as the 2M Black. Then there are the high output Soundsmiths. The Carmen MKII is about the same price as the 2M Black. I think you get more for your money with the Grado Master 3.
I prefer cartridges with fixed styli. There are trade offs in making a stylus replaceable.