Hi Beachgirl,
Post #2 here may help you identify some of the reasons that your mattress may be feeling firmer than you remember the one in the showroom feeling.
I would normally suggest following the suggestions of the manufacturer and like them I would be very hesitant to add more soft latex to your system.
I’m not sure if your box spring is an actual box spring (that flexes) or if it’s a firm non flexing foundation but if it is it may be worth trying your mattress on the floor to see if it makes any difference because a box spring can change how a mattress feels and performs compared to a solid foundation or platform.
Normally taking away the bottom layer would make the mattress firmer if it was on a firm foundation (the firm layer would have more flex than the foundation under it) but if you have a box spring then removing some of the thickness may make a difference in how much the box spring “gives” under the mattress and could account for the difference in how it feels. If your current system feels softer to you and is working well then your own experience is what counts and if a 3 layer system works then I would be tempted to use this and not “argue” with success. If you do decide to use this then it would need a change in the size of the cover.
You have the same layers as the one you tested in the showroom so the difference in how they feel could either be subjective (it’s not uncommon that a mattress feels different than what you remember testing in a showroom when you sleep on it overnight), or it could also be that the mattress in the showroom has broken in and softened slightly under the shoulders compared to the one you have (in which case walking on the mattress to help it go through it’s initial softening a little faster could help), or it could be for one of the other reasons mentioned in the post I linked (possibly your bedding or mattress protector).
While I don’t know all the details or information about your body type or sleeping style … I would be generally be very hesitant to add more soft latex to a mattress that already has a soft layer.
Phoenix