There is no 'line'...
First of all, the number of polygons isn't near as important as good texturing and the number of submodel. Every texture entails a render pass. If it has shine/glow/env maps, those entail an extra pass. If there are more than 8 lights nearby, that entails an extra pass. This holds the same for every submodel being rendered. Polygons don't matter near as much, because when a texture or submodel is rendered, the CPU passes all the polygons to the video card at once and lets it do its thing.
Second of all, even if you do draw a line, who do you draw it for? Must people with GF4 MX440s be able to run every single item in the mVPs, or can you let modders have a little bit of leeway?
Thirdly, since extra instances entail extra render passes. How many of these should it be possible to have in game at once? To answer this question, you will need to answer #2 as well.
Fourth, no-one has even tried the model in-game.
And fifth, most of the high-poly ships started at around 400 polys. A thousand extra polys isn't a big deal any more. With the last two sets of mediaVPs, there would be about 24,125 polies floating around in the first FS2 mission when the Belisarius and Psamtik arrived. To match that, you'd need about 68 of the turrets on the left. Maybe 10 ever appear at once, which is just about the same number of polygons as a single Serapis.
EDIT: Not that I'm singling you out here, this is just another in a long line of people saying "That's nice, but what about the polycount?" when 99.9% of the feedback I've seen on the forums has indicated that the biggest slowdown problem are the animated effects. I can't remember someone complaining about the polygon count of any of the fighters in-game. (Except for way back when the mediaVPs started, and someone just meshsmoothed the Perseus to 10k polies)
