Ah! Thank you, I think I understand the issue now.
ACRO mode controls the plane's roll-rate, via the roll-rate PID controller. The ground-test that you describe isn't necessarily a fair test of ACRO's performance: if you give roll-stick but the plane doesn't sense that it rolls accordingly (because it's not actually flying) then it has logic to decide how to resolve that situation. Evidently that logic has some (at least temporary) limit at less than 20mm throw, which is why you don't see full-deflection of the ailerons.
That "logic" (the PID controller + safety limits + other parameters) isn't simple, but it IS fully configurable, if you are dedicated to learning how.
But from your description of your desired behavior, it sounds like you want aggressive control of the plane when using the sticks, and for it to stabilize itself when you release the sticks. I think ACRO is the right mode for you. Would you try temporarily ignoring your ground-test and flying ACRO the way it's designed? Configure it through it's standard parameters, and see if you can get your desired behavior.
Your plane has some aerodynamic limits on how aggressive it can fly... but if you can't configure ACRO to fly as aggressively as MANUAL, then post back here (maybe also with DataFlash logs comparing MANUAL and ACRO flight?) and I (or others) can probably point you towards other parameter changes.