Hi bn880,
These are all high quality materials and there are no obvious weak links in the mattress or any “cautions” I would consider (assuming it’s a good match for you in terms of PPP which it appears from your description that it is).
These types of discounts are typically based on artificially inflated prices that nobody ever pays to create a sense of “urgency” in a customer or a belief that they are getting a “special deal” which is usually not the case.
This may be because the 2.7 lb is HR and the 2.5 lb is HD polyfoam but if the mattress you tested is a good match for you in terms of PPP then the upgrade may not have any particular value to you (depending on how each version compares when you test it). The difference wouldn’t be a significant factor in terms of the durability of the mattress and would probably have more to do with it’s performance and “feel”.
Warranties don’t indicate the useful life of a mattress and it’s likely that much of the warranty is pro-rated (there is more about warranties in post #174 here). The exchange fee is certainly reasonable.
The “value” of a mattress purchase is relative and depends on what else is available to you in your area, the mattresses you are comparing it to, and the parts of your personal value equation that are most important to you. In other words the “value” of a mattress purchase depends on much more than the “commodity value” or the “price per inch or pound” of the materials in the mattress or the mattress itself (assessing value this way would be like buying clothes based on the price per pound of the material). Value is always a combination of the “commodity value” and price, the “functional value”, and the value of all the other options, additional benefits, and features that are part of the mattress purchase relative to the other options you are considering.
Having said that … if your careful and objective testing indicates the mattress is a good match for you in terms of PPP (the most important part of “value”), the materials are good quality and there are no cautions or obvious weak links in the mattress (the second most important part of “value”) and the additional options that are part of the purchase itself compares well to the other mattresses that are in your “finalists” or are available to you then I would say it was a good value purchase and it certainly seems like a good choice to me.
Congratulations on your new mattress 
I hope you have the chance to share your feedback when you’ve had the chance to sleep on it.
Phoenix