Will I benifit form a external phono stage

yep, ph3 and p5 will reveal a bunch of rega's imperfections but you can keep rega arm if you're patient enough to adjust non-adjustable arms. also these stages "feel" what kind of cleaning did you use on your records before placing them under the needle.
ph3 is a-bit complicated on loading which is also a critical point with mc where you have to solder and un-solder different resistors to experiment. i've arranged for mine an external selector box that can be used outside of the ph3. i was intended to use this box just to measure but than solder directly the neccessary load resistor but now i simply connect this box to the arm cable(directly) and than to the phono with rca interconnects.

I love my vinyl and like you have a Mcintosh C42 preamp. I own an outboard Plinius Phono preamp (great piece) (3494) and for the last two tears have used it with a Sumiko Celebration cartridge (1500). My friend has a C42 also and was using a Clearaudio Virtuoso II mm cartridge with the built in phono amp. It sounds really good. In fact, I bought a Virtuoso cartridge and am now selling my phono preamp. I have since bought a C2200 preamp with the bulit in tube phono stage and am really pleased with that. I kept the C42 and use it in another system . I have two setups using the Virtuoso and couldn't be happier. It was 599 at the Elusive Disc with any cartridge trade in. Just wanted to give you another thought - everyone seems to have good suggestions also except parting with the C42 - unless it is to get a C2200. Have fun!

I agree with Twl that being able to optimize the cart loading will be of huge benefit. I recently got a Camelot Lancelot phonostage to replace the tubed internal one in my C-J PV-8 preamp. I am using the B-M Glider MO v.2, and found that it performed best at around 400 ohms (also, it benefitted from the use of a premium Vishay resistor - evidence that parts quality here is very important). When I compared the Lancelot at the same 47k as is standard in the C-J, there really wasn't a huge difference between the two, except for the absence of tube noise - which is where I now part company with Twl. I love tubes, but got fed up with trying to maintain the phono section and preamp with problem-free small signal tubes in the high-gain stages. Even with the best tubes I ever found, the SS Lancelot is much quieter, and tubes eventually wear out and you have to start hunting again. I'd rather be listening to music, so I'm auditioning an Innersound SS preamp as well, and could be relagating the tubes in my system to the amp alone (where I'm convinced, rightly or wrongly, that they make the biggest difference anyway).

Yes, Zaikesman, you are correct, finding good low noise and low microphony tubes for something as early in the signal chain as a phono preamp is somewhat of a pain. I like to use NOS tubes which is even harder and more expensive than the new-made types. Overall, though I feel it is worth it.
I also agree with others above that upgrading the TT would be of great benefit, more so than the phono stage. I didn't mean to ignore that point, but concentrated on what seemed to be his main point of interest.