First off, I've used a lot of carb's over the years, and a properly set up Mikuni will give you the best possible performance, period, no question.
They are also very easy to work on, and you don't need to 'pull the carb' or turn your bike upside down to make adjustments.
To answer your question, the HRS-42 is not a EPA Approved carb (like the CV) and come jetted pretty rich right out of the box... You should also realize that the big pumpers like the 42 flow very well, and I've had a number of good mechanics tell me that you would need to go with something like a CV-46 to get the same type of flow rates as a HSR-42.
In stock condition the HSR-42 comes with a 25 Pilot, and the available range of replacements is 17.5 to 32.5. As you probably know, the Pilot comes into use from idle to 1/4 throttle.
The stock main is a 160, and can be replaced with anything from 150 to 210. The Main is used from 3/4 to full throttle.
Stock needle jet is a 97, and replacements range from a 95 to a 98. The main jet is used from 1/8 to 3/4 throttle.
The accelerator pump can be adjusted, and in stock condition comes with a 60 nozzle for Sportsters and a 70 nozzle for the Big Twins. Replacement nozzle sizes run from a 50 to a 70.
If I were you I would get the model listed for the 1200 Sportsters (with the 60 accelerator nozzle), and not change anything right away. Unlike the EPA CV carbs (and EPA DOESN"T stand for Enhanced Performance Accessory) you don't need to assume that it will be jetted too lean..
You can also purchase a complete Jet Kit (PNKHS-025) which includes;
Main Jets, 2 each, 150 thru 170
Pilot Jets, 2 each 20 thru 32.5
Needles, 96, 97, 98
Pump Nozzle, 60, 70
Needle E-Ring Clips and Washers
O-ring, Pump Nozzle
The HSR also comes with a long idle adjuster, in which you can just reach down and adjust the idle speed while sitting on the bike, but that can be replaced with a regular short screw type adjuster if you want.
Anyway, good choice is going with a Mikuni. Easy to work on, instant throttle response because of the adjustable accelerator pump, and they will give performance that can't be matched by even a modified CV.
Only 'negative' thing is that although the Mikuni can't be set up for best possible fuel economy most people set them up to run slightly richer and give best possible performance. Because of that they probably won't get the same gas mileage as a CV carb.
My 2003 WideGlide went from an average of 48 MPG with a well jetted and dyno tuned CV carb to an average of about 44 MPG with a dyno tuned HSR-42. Not a big difference for the performance increase, but just something that should be mentioned.
(Almost forgot to mention, but the Mikuni 42 comes with an excellent tuning manual that provides all the information needed to 'fine tune' the carb for your specific applilcation)..