I also think that Voodoo6 for the 6k would have no made sense for the same reasons. Plus if we look at the 3dfx naming scheme since it bought STB they started with V2 1000 then the V3 1k, 2k, 3k & 3500 then jumped to V5 5x00 so there was no 4xxx cards that is were the V4 came in.
Well if compared to nvidia then we find a lot of products with different core specs within the same generations take the gf4mx for instance i had none of the characteristics of it's big brother the gf4ti and but it was marketed within the same product line also if we look at the more recent nv marketing names you get a headache ( 8800gts with 320/640mb 80 core & then 512mb G92core) generations a year apart.  They release products faster then their marketing team has time to name them this IMHO is also one of the reasons it outpaced 3dfx and the bought them out.
Also if 3dfx would have made till they released the 6k i don't think it would have saw the success it has now-a-days. Why because of it's high cost, full length pcb, no dvi or tv-out feature (typical for high-end products) and of course poor performance. You have to take into account that a lot of people "play" 3dmark and the gf had better results.
Still 3dfx lives on in our hearts and minds...