Not to mention the comb filtering that will occur.
In fact the measurements in the high end should improve if he sticks with the Neo8’s alone, especially if a line source is the goal.
That being said, the biggest pet peeve I have with line sources is that they completely lack image specificity at the expense of immense and exaggerated soundstaging. The polar responses particularly in the horizontal plane are poor compared to the vertical off axis response. Great for orchestral music though.
Combing, yes, but from what I've heard it still sounds better with the Maggie ribbon, because it's a better driver, at lower SPL's, anyway. There is some discontinuity as you cross over to the "faster" ribbon, but that can be solved by using a single pole crossover higher up, and the comb filtering can be reduced by building a new baffle in which the array and ribbon are right next to one another. A single pole Butterworth would also allow linear phase and flat on-axis and power response, albeit there would still be lateral lobing.
I don't find that my current setup lacks image specificity. The smaller Maggies do, but that's because the acoustical centers of the drivers are widely separated in the two-way models. In the Tympani IVA, you have the midrange running from maybe 300 Hz to about 2800 Hz, and then the ribbon, which is 2-1/2" from the tweeter, from 2800 Hz on up, so you get pretty good image specificity. A lateral MTM design would be even better in that regard, but I don't know of any suitable drivers since a double midrange would start to beam.
In general, I've found that image specificity can be controlled by using absorption or diffusion at the first reflection point behind the dipoles. Absorption gives you a very precise, clinical image with precise placement, but doesn't sound very real. At the opposite extreme, diffusion gives you a much vaguer image that's more like what you hear in a concert hall. Either way, I think it's a compromise, since small ensembles are better served by precise imaging and larger ones by vaguer imaging and a deep soundstage. Singers can sound too large with dipole line sources, but an orchestra will sound too small with omni/cardioid point sources. So pick your poison!
Measurements of the high end are much better with the ribbons than with the Neos alone. The power response of the Neos starts to decline at about 7 kHz, while the ribbons have essentially perfect dispersion up to 20 kHz and extension so high that Magnepan can't measure it. But mostly they sound better, unless you push them too much and then they get a hard edge. While I haven't done a final comparison, based on what I've heard, I wouldn't even think about running the Neos full range, because there's just no contest.
By off axis horizontal polar response, are you referring to crossover lobing? That's certainly an issue, but with a single pole Butterworth both on-axis and overall power response are ideal, so if you listen on axis it shouldn't be a problem. Otherwise, if the driver width is small compared to wavelength, power response should be superb and you don't have irreconcilable problems like the baffle step.
Agree that orchestral music is the line source forté, but they're also mind bogglingly good on chamber music (as heard in a hall). They have a naturalism to them on acoustic music that I've never heard from a point source. In my experience, rock recordings are all over the place, depending on the recording. And then there's the low distortion of a full height line source, and the freedom from box resonances of a dipole. Not to mention their incredible bang for the buck.
I'd be hard pressed to think of a speaker I'd rather have than a line source dipole, but that's because I listen to a lot of acoustical music, much of it recorded in (or supposed to imitate) a concert hall -- if I just listened to rock and pop, I'd probably want something like a big Wilson instead.