Since expansion chambers are designed to give the Co2 flowing through your marker more space to turn into a gas, this will do nothing perfmornace wise towards running HPA through it. It will act as a vertical gas-thru grip.
A good regulator for just HPA is the Custom Products Inline. Very cheap, very compact, and very good.
But for both Co2, and HPA, flatlander called it with the Palmers Male Stabilizer. This is also a very good regulator when it comes to either air source, but most will find it a little expensive at $90 - $100.
I'd suggest Macroline also. Just like Barber, I only hear bad things concerning Microline.
I think you may need some kind of HPA kit for your A-5 if you want to go HPA. I'm not sure exactly. But a regulator with HPA will be a good first step. This will help regulate your marker, and make it more consistent.
You will get way more shots from a 68/4,500, than a 68/3,000. Totally worth it IMO.