Hi emurphy,
1.5 lb polyfoam would be risky in a comfort layer unless it was either two sided mattress or was in a very low cost mattress where lower quality materials with less durability are part of a lower budget choice. The weak link of a mattress though is in the upper layers not the base layers so for most people of “normal” weight I would consider the 1.5 lb polyfoam to be suitable in a lower budget range where there were high quality materials in the comfort layer. If you were in an above average weight range where you were more likely to “go through” the comfort layers and compress the foam underneath it more then I would certainly consider a higher density base foam.
Everything is a tradeoff and it depends on your specific needs and preferences and which parts of your personal value equation and the many tradeoffs involved are most important to you.
Phoenix