Theoretically perfect is a lot more understandable to me than putting a bunch of variables into the equation. After assuming scope A has optical fault X and scope B has optical fault Y, it approaches blue smoke and mirrors, and you can skew the data to favor either viewpoint at the extreme end of the discussion.
Oversimplification is always easier. That doesn't make it
good. Understanding actual telescopes inevitably requires
inclusion of actual factors. It may surprise you, but it is
not complicated at all. Aberrations and other wavefront
deformations, c.obstruction, seeing - they all can be with ease incorporated into MTF as degradation factors. It results in a realistic MTF.
If you're saying that a larger aperture scope with a poor mirror won't beat a perfect smaller refractor, that's all well and good for the poor soul who deliberately goes shopping for a scope with a poor mirror. I don't see how it helps the rest of us.
Poor mirror (or optics) is not a prerequisite for poor telescope or performance.
Nobody goes shopping for a poor telescope, but not a few people do get it, sometimes with
claims of good or even excellent quality. Larger apertures
are prone to larger errors from all sources; even a perfect
primary is not a promise of good performance.
Some of routine extra error sources of a large
Newtonian are primary deformation induced by its cell, thermal imbalance,
miscollimation, diagonal inherent
error and cell/thermal deformations, seeing,
and, of course, central obstruction. Let it be 1/8 wave
p-v mirror (0.95 Strehl) to begin with. We can say it is
a very good mirror. What will, say, 20% c.obstruction do?
We can quantify this effect with the lowering of the energy
encircled within the Airy disc, given by (1-c^2)^2, "c"
being the c.obstruction relative size. In this case it gives
0.92 which, for the left-hand side of MTF graph (which is
what matters for planetary observing) can be taken
as an MTF degradation factor. Combined with the correction
error, it will result in ~0.87. Although not a Strehl,
strictly talking (since it doesn't all come from wavefront
deformation), it can be looked at as sort of it. It says
that average contrast loss over the range of MTF frequencies
is ~13%, resulting in performance level comparable to
~13% smaller (linearly) aperture.
Let's say that miscollimation is 1/20 wave RMS , and that cell induced and thermal error are 1/40 wave RMS on both
primary and diagonal. Let's throw in 1/50 wave RMS on each, primary and diagonal, on account of local surface irregularities. Since these are mostly uncorrelated errors, we can use the square root sum squared to obtain likely cumulative error. The square root of the sum of all the RMS
errors squared comes to 0.076, or 1/13.1 wave RMS combined
error. From S~1/2.72^(39.5RMS^2), the appropriate Strehl
comes to 0.79. This is the next degradation factor to add,
and it lowers the system Strehl-like figure to 0.69.
Adding a very moderate 1/14 wave RMS seeing error results
in 0.82 degradation factor, lowering system's figure to
0.57. Starting out with a very good mirror, we ended up with
a telescope performing at ~0.57 Strehl level, a 1/8.4 wave
RMS level (from RMS=0.241sq.rt(-logS)), an equivalent of
1/2.5 wave of spherical aberration.
If similar calculation would be done for a 4"-6" apo
with 1/8 wave objective, all the error sources would be considerably lower. Thus the nominal aperture difference
would shrink down to a considerably smaller actual
difference. The point is that c.obstruction alone,
even if we put it at 35%, would play only a small part in it.