Output Impedance and Speaker Impedance

Looking at the spec for the MA7900 I'd guess you're probably okay. I don't know the mechanism they use to achieve 200W into all different impedances.
The difference between the speakers' sensitivity is 4dB which means the less sensitive speakers will need 1.6x the voltage to achieve the same SPL.
For the amplifier to output 200W into 4Ω then the RMS voltage needs to be 28.3V drawing about 7A. For 8Ω the figures will be 40V drawing 5A current. So assuming the amplifier controls the output voltage to match the output levels then we might expect that the 8Ω output puts out 40/28.3 or 1.4 times the voltage of the 4Ω output. 1.4 x the voltage equates to around 3dB. So this would explain the better level matching.
But now when you run at full volume you're putting 40V into 4Ω which will draw a current of 10A. In a lot of cases that would be a problem and you could end up with overheating but in this case the amplifier is capable of running with a 2Ω load so the chances are you are okay. If you put a 2Ω load into the 8Ω output you'd draw 20A which would likely put the amplifier into protection mode but is not worth the risk of trying.
Another way of achieving the same (or better) result would be to put a simple voltage divider (two resistors) across the inputs of the higher sensitivity speaker to reduce it's sensitivity by 4dB. You may be able to buy a 4dB attenuator to do the job for you.

Hope that makes sense.

You can really take this as far as you want to...
1) You say you know what you like (which is great) so if you like the sound you're getting stick with that... everyone seems to agree you're not going to damage anything.
2) You can make up a lead to attenuate the Tritons, the simplest way to do this would be to go back to your starting configuration:

Mac MA 7900 (200 watts) that I use to power Aerial Acoustics 5T's and a MC275 (75 watts) powering Triton 2+'s .

And adapt (I couldn't find an off the shelf solution) a pair of interconnects to connect between the MA 7900 and the MC275. To do this you'd need a handful of components, some basic tools and rudimentary soldering skills. The advantage of this is that you can experiment with different levels of attenuation. If you're interested in going that route I can elaborate.
3) This is probably heresy here but you could use DSP... that would let you completely control the output of the two sets of speakers by controlling both the signal level and the frequency ranges sent to each speaker. So you could roll off the top end of the Tritons and the bottom end of the Aerials. It's easy to sniff at DSP but Siegfried Linkwitz (he's got quite a good CV) used it in the designs for his reference speakers.

If you're happy with the sound you've got then definitely go for option 1.