FX T12 whisper synthetic

I just ordered a used FX T12 whisper synthetic .22 from AofA. I did a little research and they originally came with a Lothar Walther barrel. Now that they come with the smoth twist barrel I was wondering if I so chose, could I install a smooth twist barrel after removing the Lothar barrel? For the price I paid I am fine with whatever barrel is installed (one really can't go wrong with a Lothar, right!).

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 Glad I can help! This forum has been a godsend for a lot of people.

My Whisper doesn't have a pressure gauge and I'm not the best about counting my shots. 

It shouldn't be that big a deal since I top off my airguns every time I use them, but it is. The Whisper is my only gun that doesn't have a manometer. The others have spoiled me. 

AOA has them and they are easy to install. This is a pretty simple rifle to take apart.

https://www.airgunsofarizona.com/pcp-accessories/fx-air-tube-gauge-kit/
 
This rifle is a shooter! How would I tell a Lothar Walther barrel from a Smooth Twist? I'm sill leaning towards a regulator with a 45cc extension and maybe the guage. I actually get between 30-40 shots between 850 and 860fps. If I go beyond that, it starts falling off from 850 to 820 in the next ten shots. I just wonder how much better it would do with the extension and the regulator?

Christopher