To be honest, there's calls for both schools of thought, with only a couple of hundred polies on the older models, there's already an awful lot of artistic license going into what is being made. I still think it's a kind of a complement to the skill of the modellers that when people look at things like the HTL Loki they see the 'old' Loki inside it.
I'm pretty sure FS2 does backface culling when it renders anyway, which means the most polies from a 5000-count model you are going to see is about 2500, and most hardware can compute the matrix-based rotations and deformations at millions of polygons a second. To be honest, with the poly count itself, you can go pretty nuts, with detail boxes on large ships you can go totally nuts.
That said, as has been pointed out before, most fighters aren't actually in view up close for all that long, and particularly in the case of the Shivans, details are actually pretty hard to make out, so sometimes it's worth bearing in mind that just because you can do a thing, it must not always follow that you must do that thing.
Most HTLers on here do it because they love it, they aren't thinking like game designers, they are thinking like modellers, that has resulted in some of the best in-game models I have ever seen, revisualised from, in some cases, 100 poly 'blobs', I don't have problems with speed on the new computer, but I can understand, however, how people with lower-power systems might be concerned.
The trick is not in the Lod 0, oddly enough, a squad of 4 HTL bombers attacking a capital ship might contain roughly 40000 polygons in total, even a medium-level HTL card could work out the on-screen positions of those polygons faster than it could transfer the texture data, especially if the Capship uses multiple textures. Textures have considerably more impact on performance than polygon count.
Let me put it this way, in Oblivion, there are 20,000 polygon boulders, not mountains, just boulders that you can jump on.