Minolta question: Sigma 400mm f/5.6 versus Minolta APO 200mm f/2.8 and 2x APO converter

Greetings.

Hopefully this question can be answered by a fellow Minolta nut or
anyone else with broad knowledge of apochromatic lenses versus the
standard ones.

I have a Minolta 7xi film camera, but I also have an RD-175 digital
which uses the same Maxxum lenses (and I plan to buy the new D7 which
also is designed for these lenses).

(For background, the RD-175 body features a lens element that reduces
the magnification factor to around 1.25x, whereas the D7 will
experience a full 1.5x magnification between the lens image and sensor
size.)

I currently have a Sigma 400mm f/5.6 lens which produces very
acceptable results.

Today I found a pair of lovely used items at a local camera shop. One
is a Minolta APO 2x Teleconverter-II. The other is a matching Minolta
High-Speed AF APO 200mm lens (from their family of high-quality,
white-bodied gigantic lenses).

I bought the converter and am deeply interested in the lens.

On paper, the duo will match the specs of my existing Sigma lens
(400mm total magnification at f/5.6 due to the 2x conversion).

But will the Minolta APO pair still have a significant edge over the
Sigma in terms of overall quality in most conditions? Apparently the
focusing is quite speedy on the stubby APO, too.

The price on these seems to be right -- $139 Cdn. ($110 US) for the
teleconverter and $499 Cdn. ($400 US) for the lens. So I'm sure the
teleconverter was a good purchase. I'm just wondering if many people
would consider the quality of the Minolta lens to be a reasonable gain
for that expense.

I have a 100-300mm f/4.5-5.6 Maxxum xi lens, so I've already got the
200mm focal length covered, but the APO would obviously be much, much
brighter.

I will then need to decide whether to (a) sell the Sigma to offset the
purchase price of the 200mm APO or (b) keep it for those few occasions
when light is sufficient to couple it with the 2x for an 800mm f/11.6
combo.

Thanks in advance for any opinions offered!

HD

P.S. -- now all I need is a Maxxum 50mm f/1.4!!! Anybody need an
RD3000 with three lenses, to help pay for the adoption of a D7? Heh
heh...