The SSi/SSp 15's will get plenty loud.
As for the looks of the SSp's, they're UGLY!, even the cone/dust cap turns me away. I'm glad I have my SSi12.22's. Any mass added to the motor structure will aid in cooling, but it doesn't seem that those would work enough to make a difference. Even though my SSi 12's are rated for 750 wrms, I wouldn't think twice at giving them 1000 wrms a piece as are the SSp 12's are rated for.
That pic made me LOL too. Like I said I have 0 experience with the NBS GM's. I just know that when I looked at them there was virtually no room in there.