It's looking awesome so far - I love the greeblage. Nice work.
You may already be aware of this, but just in case;
It's pretty obvious that with each new subobject being rendered, you're slowing down the rendering of that ship. However, what's not as obvious is that while rendering huge numbers of polys might be a performance drain, it's a considerably bigger performance drain to render the same number of polygons if they're split over multiple subobjects.
What this means is that you have to carefully plan your detail box sizes and positions if you want to really benefit from them. You want as few of them to be visible at once as possible, but you want the ones that are visible to reveal a lot of detail without rendering stuff that's so far away that it becomes invisible anyway. How this is decided varies considerably based on the ship in question, but it comes down to: make them too big and you end up rendering a lot of detail you don't need to, effectively canceling a lot of the performance gained from the use of detail boxes in the first place, but make them too small and you'll end up having to display too many at once which could also cancel out the performance gain, and possibly add some more drain on top of that.
It's quite the balancing act really.
Oh, I don't know what you're actually planning with destroyed meshes, but don't make destroyed meshes for anything that you wouldn't expect to be destroyable like for example, the dome things, turrets, engines etc. Otherwise it'll be a LOT of work for effects that are very rarely seen in game.
Go crazy with the debris mesh for sure - the results are very worth it, but destroyed subobject meshes that are there for nothing but the sake of having destroyable stuff on the hull just...isn't.
Anyway the ISD is still my all time favourite ship design ever and I have to say this one is looking great.
