I'm afraid I don't understand why people thing everything in s3 should be a tank. What's wrong with a glass cannon? Is there are reason balance-wise this change should be made rather that simply: "its in s3"?
Not exactly a balance reason, but a gameplay one (o.O) It's generally much better to build an acid tube, even when a hive could be built:
Reason 1: Acid tube has the same or better defense numbers (same HP, smaller profile).
Reason 2: Acid tube builds faster.
Reason 3: Acid tube damages several people at once, constantly, unlike the still lethargic attacks of the hive.
In general, there isn't that much reason to build a hive as compared to a tube (the exception being attack range, which doesn't help all that much considering aliens try for compact base locations and just-around-the-corner building.) By suggesting this, I'm trying to make the hive a bit more appealing to build when it becomes available. The fact that it deserves something because it's S3 is the weakest of my arguments, ironic that you would cite that. Still, even that is a reason. Aren't higher level rewards supposed to have better appeal?

Additionally, I disagree with you that adding 25-75 HP to the hive would make it a tank. That minor addition would give the hive a better survivability chance, something to make it worth building over a tube.