Will I benifit form a external phono stage

In two words or less- probably yes. Conventional wisdom is that, all things being equal, a lower output MC will outperform a higher output MC with a small part of the improvement offset by the higher gain (higher noise)needed.

With respect to phono pre-amps, you will probably hear a significant improvement with the ones you mention over the inboard unnit on the Mac. However, I would recommend that you look into an entirely different pre-amp such as the Audible Illusions with the Gold Card phono section. Superb tubed pre with superb tubed phono section. About $1500 for a good used one. There are several other great pre's with excellent phono sections out there, too. You'd improve your ENTIRE system going that route, rather than just part of the analog portion.

Regards
Jim

IMO, you will get an improvement. The two phono stages that you mention are very good ones for the money. Nothing really wrong with your HO Benz, but, in general, LO will be a little more detailed due to lighter moving parts. Being able to "load" the cartridge properly with your new phono stage will help also. Regarding the mixing of tube gear and SS gear, many people do that to get some of what they like about tube sound combined with what they like about SS. Particularly in the phono stage, since that is very early in the signal chain, the tubes will provide a very nice smooth signal for the SS stuff to work with. I expect that you will like it. You might like it enough to change the rest of your gear to tubes.

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.