Will I benifit form a external phono stage

I believe you would do better to improve your TT and arm.

sometimes there are built-in phonos that are so perfect but too rare. separate box is more sonically beneficial. ph3se is ideal with medium output carts. bat p5 can go as low as .3mV
both of them have excellent performance.

I agree with all the above comments. Being a former rega 25/600 owner and a former BAT P5 owner, that's a great combo, however, your TT is quickly going to be the weak link. In fact, and this is debatable I'm sure, but you may be better off with a better table/arm/cart now, then step up on the phono stage. Maybe keep the arm and go with a Aries, or Gyrodec? Or if you want to keep the rega, tweek it with a new counterweight, platter mat, and speed control.

I loved the heavyweight from Express Machining, the ringmat, and I have heard on Audio Assylum that the VPI motor controller is a huge upgrade.

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!