Well I'm not sure what you're getting at drk. If you want to talk about a duct's ability to handle static pressure then sure. Even flex can handle up to 10" W.C. long term without blowing up. Since most never see much more than one inch W.C. in the worst of residential cases then there's a lot of headroom in that regard.
But of course no one is talking about that when they suggest that ducts can't handle a certain amount of air. They're talking about the fact that the ducts may or may not be ideally sized to allow the necessary amount of airflow when matched with a certain blower, coil, outdoor unit, etc. Surely you know that. And if you don't then you perhaps should follow some of your own advice.
I think drk is right using the same line of thinking you should go with a 5 ton 18 seer condenser, it should match up well with the 20 year old 3 ton 6 seer coil you probably have in the house, 6 in to 18 mean it will cool for 3 times less than if it was a 6 seer but even better because it is a 5 ton which means you will have 2 extra tons of reserve cooling for when it's hot. You know the 13 seer really doesn't fall in here anywhere because 6, 8, 10, seer that you might have doesn't divide into 13 any which way you try.
If your worried that you existing duct can't handle the "extra air flow" to slow down the air flow just run your filter dirty or when you install a new one only remove half the plastic wrapper this will help reduce the air flow and extend the life of the filter. Once half the exposed half of the filter is dirty the air flow will be reduced, remove the other half of the plastic and re install it on the dirty side, if the dirty side is clogged no need to reinstall plastic. Using this method you should get about 2 years out of a standard glass filter.
Unless you change the indoor coil you will never see the benefits of a 13 seer condenser, the indoor coil is probably sitting at 6 or 8 seer. The industry only figured that your system would last around 10 to 15 years when it was manufactured so you've got your monies worth. With 20 years on the system do yourself a favor change it all out and be comfortable for another 10 to 20 years.
While I appreciate sarcasm as much as anyone yv, yours had no substance. If any of it was for my benefit then please point out any factual errors on my part and I'll gladly recant. I don't think you can. Not everyone drinks the new equipment hustling Kool-Aid.
Thanks for the good advice.
I cancelled the replacement this morning (the guy who didn't even do a leak test) and called the company I've dealt with in the past (which was too busy to come out yesterday). I've taken your advice to slow down (and do it right)- though it's tough to overcome the impulse to get cold as quickly as possible.