I think before you start tweaking someone else's rules it's best to understand their reasoning behind values, what overall influenced them what is abstracted or not considered at all. Much of rules writing is subjective to the author based on his experience, knowledge, and bias. I think that goes for all of us, me too. It would also be good to understand why the designer didn't implement your suggestion or how it is maybe already covered or abstracted.
My opinion is that in a buttoned-up tank, you are not "blind" as many people say. My experience sitting in commander locations buttoned up is that your Situational Awareness in the 20-degree aspect you are looking at is pretty good out to a limited range. But almost completely blind in the other 340 degrees so you need to swivel. If you are advancing in formation with infantry support and flanks secure all you really need is that frontal view. However, without enhanced optics, your sight distance could be limited.
Most Allied tanks had roof-mounted panoramic periscopes with a field of view up to 60 degrees and 4x power. The T-34/85 and Sherman had one for the gunner and commander. The British and Russian periscopes had the capability to quickly look directly in the rear without the crewman turning around. Optics quality matters too. Early war Russian observation optics were pretty bad.
Each vehicle would have a sector to observe and fire into so a 40-60 degree field of view should be more than adequate and hopefully overlap. On the other hand, most tanks are completely blind if you are within 20-30 yards of them.
Then there is what definition of buttoned up and unbuttoned are you using. Unbuttoned can be anything from the commander standing tall with his entire torso exposed to the commander popping in and out taking a quick peek every 5-10 seconds. If the tank is engaged and shooting the commander is probably going to have tunnel vision to the front and be unaware of everything else around him.
In tanks with one or two-man turrets, the commander was the gunner or loader so when engaged he was generally locked down and would have a lower Situational Awareness than normal. That's about a worst-case scenario as you'll run into except maybe the French one-man turrets.
I think the biggest advantage of unbuttoned is enhanced Situational Awareness that should cover your frontal 180 degrees. Some "experts" have said being buttoned up reduces your effectiveness by 40%.
As a result of rounds exploding in the vicinity, one doesn't hear the gun's report at all in the tank. It is quite different whenever the tank commander raises his head occasionally in an open hatch to survey the terrain. If he happens to look halfway to the left while an enemy anti tank gun opens fire halfway to the right, his eyes will subconsiciously catch the shimmer of the yellow gun flash. His (the tank commander) attention will immediately be directed toward the new direction and the target will usually be identified in time. Everything depends on a prompt identification of a dangerous target Usually seconds decide. What I said above also applies to tanks that have been equipped with periscopes."
Quote from Otto Carius Tigers in the Mud
Regarding infantry and tank assault I think you need to do more research.
Hey, it's a game, if it feels good do it.
Wolfhag