Here's what I think:
Removing the 5 from play in a 2 deck game increases the true count of the played portion by 0.5. From what I've heard, each TC point is worth about a 0.5% player advantage. So if the starting house advantage is less than about 0.25%, then the BS player does have an advantage! Albeit, not very much advantage.
Since the 5 is a particularly important "low" card, I would imagine that the advantage over the baseline edge is slightly higher than 0.25%.
On the other hand, I don't think that cutting an ace into play has as big of an effect (in terms of flat betting). Suppose the game features 75% penetration. By ensuring that an Ace is played, you're increasing the expected proportion of small cards in the non-played portion. The expected true count of the non-played portion will be -0.5. To balance this (the true count of both decks must be 0), the expected true count of the played 75% portion is +0.5/3 or about +0.167. This gives the player less than 0.1% advantage over the starting house advantage, which is probably not enough to flat bet with an advantage.
Interestingly, the weaker the penetration, the better the true count in the played portion (for the cut ace example). For example, if penetration is 25% and you manage to cut the ace into play, the unplayed portion's true count is still -0.5% but the played portion's true count reaches +1.5! Even with good penetration, you can make this effect work by cutting the ace into the first 25% of the shoe, flat betting for that first 25% and sitting out the rest of the shoe.
However, (and I know this isn't what you're asking, and I'm sure everyone already know this) you should have a good idea of where in the shoe the ace was cut to. With that info, and the willingness to not relegate yourself to flat betting, you can and should raise your bet when the shoe is dealt to the region where the ace is sitting.
Thank you for the thought exercise! I'm sure someone else will chime in and correct any mistakes I made
