Every lawnmower is going to be slightly "underclocked." 29 CFR 1910.243(e)(2)(d)(ix) sets the maximum mower tip blade speed at 19,000 ft per minute. For a 21 inch blade, that works out to be just slightly over 3450 RPM. Every manufacturer is going to set the engine speed somewhere below that (around 3200-3400 RPM). No manufacturer is going to take a chance on violating the code and risk liability. On the other hand, a manufacturer isn't going to set the RPM so low that their machine is not going to perform well--just not good for marketing.
My guess is that as part of "prep and set up" (which any dealer is supposed to do), they may check the RPM for factory spec and adjust to factory/manufacturer spec. if needed (doubt that they come across many/any out of spec as RPM would likely be part of standard factory quality control). If you find the mower not working as intended, most every dealer will adjust the RPM to factory spec if it is out of spec., if needed, as part of service under the warranty Sounds like a bit of sales puffery to me.
Keep in mind, most, not all, dealers try to distinguish themselves from the big box stores by personally standing behind what they sell and giving better and more timely warranty service than you can get from the big boxes. (When I bought my HRX from the big box and returned it due to a fuel leak, they just had me grab another off the shelf--not much of a hassel.) In other words, you spending that extra $100 based on the reputation of the dealer to provide superior service--so check his reputation and decide if it's worth the extra money for your peace of mind.

"In other words, you spending that extra $100 based on the reputation of the dealer to provide superior service--so check his reputation and decide if it's worth the extra money for your peace of mind." Good advise. I can tell you that many mowers that I work on- I am the first person that ever puts a tach on them and adjusts the engine speed. That is one sold through agent orange or the local dealer. Commonly a 21" is set at 3150 which allows for ~ 10% governor speed allowance. I get the feeling that these GCV engines really don't want to run at a much higher RPM anyways.

The HRX I bought from HD runs at 3060 RPM. Factory manual says maximum governed RPM is 2950-3100. I used a Tiny Tach to measure the RPM.

There are many that are 2650/ 2850 - it is amazing that they leave a servicing dealer at that speed. GXV's don't like much over 3100- you are correct. I don't adjust them above 3150 no matter what they are on.