When is a variable-speed compressor HVAC unit worth the cost?
The test procedure for determining the SEER is quite elaborate, and involves measuring the efficiency at a variety of simulated outdoor temperatures. I'm not sure what the basis for the sales rep's statement was, but without more information (i.e. specific numbers) I wouldn't put too much stock in it. The SEER is not just calculated at a single fixed situation.

As for whether it makes sense, that depends on your climate, the size of the area being cooled, the local price of electricity, etc. I would just use one of those calculators like you've linked to to determine the value, but make sure you get accurate numbers for all the inputs. The cost of electricity varies widely throughout the US, so get a current number for your area in the summer and then maybe round up a bit to account for the gradual rise in prices.