Okay, I admit I didn't realize a zone got to apply your implement bonus every round. I'll buy that. I don't accept that 7.25 value, though; you either deal ~29 damage or 14 damage, depending on your attack roll.
I'm basically taking the averages here. Yes, it is 29 damage or 14 damage, but assuming a 50% chance to hit, that is an average of 22 damage.
Given... but fireball should be comparable to Stinking Cloud -- not Cloudkill!
But again - it isn't comparable to Cloudkill. It hits less enemies for 2/3 the damage, assuming you can't hit the enemies additional times with Cloudkill (which, generally, you can.) Cloudkill is superior in several ways - not enough to make Fireball an irrelevant spell, but enough to clearly be several classes above it.
Now, I admit that the real relevant point you bring up is how good Stinking Cloud is (and it is pretty darn good). But the difference in area of effect is really a big deal, and that seems to be what you aren't weighting heavily enough. Stinking Cloud covers 25 squares; Cloudkill hits, what, 121 squares? Yes, it adds some hindrance if the enemies are intermixed in your group, but since you get to choose when you cast it, the benefits of the extra size will greatly outweigh any negatives.
Well, 35 is kind of a bogus number there. It's either 45 damage or 21 damage, depending on the attack roll.
Again, I'm going by average damage based on a 50% chance of hitting, simply for ease of comparison.
And again - Acid Wave, a spell of equal level to Cloudkill should be comparable to it. Hitting less enemies for more damage (assuming no potential for a second hit from Cloudkill) seems a fair trade-off.
I'm not underestimating the burst size; what I'm saying is that Stinking Cloud is (A) easier to fit into the average battle without poisoning allies; (B) nearly as good at sticking to your enemies, via moving faster; and (C) deals exactly the same damage.
If you drop Cloudkill at the start of a battle, you are almost guaranteed to hit the entire group of opponents. Stinking Cloud, you will likely hit half of them. I mean, you are free to think that having an 11 x 11 area of effect is not very helpful, but if that is your opinion, I don't think anyone will be able to change your opinion in this matter. That sort of area is enormous on the field, and very powerful.
Now, the one good point you do raise is that Stinking Cloud itself is quite good for its level. But by level 19, getting the extra coverage from Cloudkill's size is a very big benefit, and what makes it so fearsome on the field.
But it's not significantly better than keeping old Stinky. Maybe it's true that Stinking Cloud scales very nicely with level -- a 19th level spell ought to be better than a 5th level spell cast by a 19th level wizard!
And, again, it is, due to the size. Since you don't consider the size a benefit, there really isn't any way to convince you that Cloudkill is better, so I'm content to let the disagreement end there.