Hi Alpharalpha.
I take it back … It’s not impressive! 
But I am glad that the manufacturer you worked with was so responsive and went out of their way to help find the un-zoned L&P you desired. Provided that your DIY core support is appropriate for your needs then a topper could be a good option for fine-tuning the comfort.
I understand why you’d be “divided” between the two options and I’d try to see if the manufacturer can find a way to further help you maintain the ability to customize your DIY in case your desired construction turns out not to be quite right. I agree that whichever direction you chose to go, you’d have better chances of success by relying on their advice as they’d have much more experience to guide you to the best possible outcome. I’d also make sure to look at both options once more in the light of all the parts of your personal value equation that are most important to you (including the price of course and the options you have if your choice doesn’t turn out as well as you hoped for).
I’ll be interested in learning about your final decision.
Phoenix