Gary,
The magnification for 6x9 to print an 8x10 is approximately 5x +/-
This is well under the 11x that Fred specified as his transition point to a 6 element lens.
I do not have the specs available, but if the 6 element lens is designed for 10x and larger, and a 4 element lens designed for 2-10x, the 4 element lens might actually be better than the 6 element lens when printing at 5x. It is all about working withing the design specs of the lens.
Then again, I used an el-Nikkor 50/2.8 (a 6 element lens) for almost all my 35mm printing, from small (wallet size) to 16x20 this is approx 2x to 16x magnification range. But 35mm film is where you have the issue with larger magnification range. The issue gets smaller as you go up in film format size, with a corresponding reduction in magnification for a specific print size. On my upper end, to print an 8x10 from a 4x5 negative is only about a 2x magnification, vs. 35mm which would be an 8x magnification.
To have the latest and greatest, I would get the latest Componon-S or Rodagon. But again, for a 5x enlargement, do you really need that extra optical quality, at a higher $$$ cost? Some of those large lenses also need special lens board with a larger hole for the lens, more $$.
But let us step back a bit. To take maximum advantage of the lens, you will need to use a glass carrier, to hold the negative FLAT. If the negative has a curve in any way, while in the usual glassless carrier, you have already lost some of the quality you are squeezing for. And from my limited experience, using a glass carrier was a PiA, because of the issue of keeping the 2 glass plates absolutely clean. And most darkrooms are not dust free like computer clean-rooms. I could see dust particles floating in the light from the lens to the easel.
On the other hand, with prices as they are now, you might just get both the Kodak Ektar and a modern 6 element lens.