+5 sight will help when the tank rushes in first, like all tank's youll need a spotter to shoot further. It's not completely redundant, actually it helps the enitre team a little.
Does it help your team as much as Improved Barrels? Would you trade that +5 sight for +5 range on your Pershings, Shermans, Crocodiles,
and M10s (not to mention every other tank you get from the Reinforcement packages)?
You've said what I've been saying all this time again. You need a spotter regardless.
On the defensive, people don't exactly place their M10s on the front lines. They use Jeeps or Rifles as the front, to spot ahead. The sight bonus is useless on the defensive.
On the offensive, the situation is somewhat different. As you've stated, the +5 sight helps when the tank rushes in first. However, only the +5 sight helps. If the tank rushes in first (as per your scenario), it's spotting the enemy first. However, the sight bonus is still 5 units less than the M10's original range. In short, the M10's extended range doesn't do anything, only the sight bonus helps. If you are advancing ahead with infantry before armor, then the sight bonus is, once again, worthless.
So, in short, you are getting a T3 that adds +5 range OR +5 sight in the best case scenario, on one vehicle model. That isn't exactly T3 worthy. Improved Barrels grants the range bonus to all your tanks, including heavies, mediums, and anti-infantry as a T2.
I'm done here. I'll do my best to avoid this topic.