Thanks for the analysis, guys! This is exactly the kind of info that I was looking for
I think I may still end up converting 1-4 Heavy Destroyers, partially because the conversion I have in mind is a modular one (and not the one you probably are thinking... I'll share my secret plan soon

) Adding the Heavy Destroyer option wouldn't take that much effort, and I enjoy the challenge. Again, you can keep an eye out on my P&M Blog for that and other conversions and so forth:
http://www.dakkadakka.com/dakkaforum/posts/list/369482.page
Another reason I might still do them is that I have about 5-6 on-sprue Destroyers, and I like the design of them, and am enjoying the various conversion possibilities (like my above-mentioned Destroyer lord). I might convert the Destroyers to Canoptek Spyders, so that I can save my Spyder/Wraith duo-kit(s) for the Wraith option, since I'll need those nasty mofos for a scary
CC squad to accompany my Destroyer Lord.
Any further thoughts on Heavy Destroyers, etc. still welcome, and thanks again Dakka!
-C6