Will I benifit form a external phono stage

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.

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.