Is going from 16" Dob to 17.5" worth it?

I thought I had solved aperture fever by getting the biggest mirror I could get and still keep my feet on the ground, but here I go again.

My choice is between sticking with a 16" above-average 1.625"-thick f/4.5 mirror with so-so coatings, to using the same structure for a new 17.5" f/4 1"-thick mirror with excellent figure and coatings. Considerations are improved cool-down time, lighter weight, better resolution, and 20% light gain on size alone, probably more like 30% factoring in the quality of the mirror. There's a fair amount of tinkering with the hardware to make it work, but I live for that kind of challenge.

My general rule is that one has to move up 2 aperture classes to get a "Wow! That was worth it!" difference. That would put me into a 20" as my next scope. Any opinions?

Not much difference aperture wise; but you could improve the optical quality and it would perform better.
From 16" you would need about 20-22" to see some real differences. But even then it is hard to say it would be WOW!

That is a tough choice. The net light throughput gain will result in a slight improvement to image brightness and slight increase in limiting magnitude but the image might be considerably better due to the improved coatings, quality of figure and aperture. I don't know that the scope will be any lighter weight-wise, but if it is due to improved lightweight construction techniques, it will also be a bulkier overall too. It definitely won't be a hugely noticeable change at the eyepiece but if you like to tinker . . .

My general rule is that one has to move up 2 aperture classes to get a "Wow! That was worth it!" difference. That would put me into a 20" as my next scope. Any opinions?

I don't think going from a 16 inch to a 17.5 inch is going to result in any WOWs or even wows...

But it is my opinion that one does not move up in aperture to get that WOW, the WOWs are done with soon enough, rather one does it to have a better tool. If the 17.5 inch were a better scope and you are willing to spend the money to get a better scope, go for it. But it seems it's the same scope, different mirror. I would stick with the current configuration and if the mirror needs upgrading, get this one refigured.

Jon