deadduck is right, a turn of the screw and you have 2388 hp.
As far as the differences, there have been multiple updates by year to the axial flows. Go to http://www.hoober.com/ag/axial_flow_history.asp for a complete rundown. I run turned up 21s alongside multiple different 23s and the single biggest performance difference probably came with the AFX rotor in '03. Not much different in wheat, but in tougher conditions it is much smoother. A similar setup can be added to a specialty rotor from a couple of manufacturers. Stewart Steel makes one http://www.stewartsteel.com/ag/ag.html and Loewen http://www.loewenmfg.com/006.html makes another. I like the looks of the Loewen kit a little better, I believe it uses the OEM AFX impellers. I hope to slip one in before soybean harvest to try.
I've ben running our 2188 with a half turn on the fuel screw for 5 years or better now. Thats about 50hp. The only problem is cutting wheat in the summer when its hot. Got to keep the radiator clean or it will overheat.
Yeah, Big A in SC KS we just can't get away with 1/2, much as I'd like to. About 4-5 pm when it hits 105 she just gets to warm even if you clean the radiator every morning. It depends on which direction you are cutting vs the wind direction, but it is frustrating so we backed them down a smidgen. We run probably about 3/8 of a turn. It never occurred to me until now that a guy could probably give it that extra twist for cutting green stemmed soybeans in the fall though.
Yeah in the fall I have no problems cutting beans on corn. Just in the summer cutting wheat. You know when its 90-100 degrees close to 100% humidity. I wash the radiator out every week at least. When your cutting with a 30 ft head in thick wheat averaging around 100 bupa going 3.5-4.0 mph. Works her hard. Sometimes 3.5 mph is all I can get. Last year for example we rented a AFX 8010 with a 30ft head to help get done wheat and I ran 5.0 mph all day long with no problems. Now in the fall when its in the 60s cutting 50-60 bupa soybeans I can go around 5.0 mph, and picking 200 bupa corn with a 8 row head about 5.0-6.0 depending on the moisture. Oh well that is farming.....