Every one who is interested in the A-20 should read this piece (.PDF format).
http://legendsintheirowntime.com/A20/A20_draft_2.pdfNote that for normal maneuvering, the A-20 was rated for +4g, -2g. However, the ultimate limit is a whopping +9g, and that's with standard design safety margins (typically no less than 2g). Of course, this assumes no ordnance on the wings.
Operational dive speed was rated at 120% of max level speed. For the A-20G, that was about 396 mph. Yes, it could be dived a bit faster without drama, but please remember that safety was top priority and the operational margins were well below what the aircraft could actually do. In game, I use 400 mph as my redline.
As to whether or not the A-20 was used in real life the way some use it the MA, the answer is not very often. However, pilot instructors would often demonstrate the A-20's maneuverability against fighters. P-40s were the most common adversary, and the P-40 jocks found the Havoc more than a handful.
Anyway, read the piece and realize that the A-20 was designed as a multi-role aircraft, and good maneuverability was designed in from the outset.