A Mak has more like >80% if taken everything into account and with modern coatings.
And this still gives you an aperture of 150mm for your 180mm Mak: sqrt(170*170*0.8) . 170mm is the effect of obstruction.
And your 5" apo has at most 95% light grasp if you are lucky (so 120mm effective).
Sure the gap between 120mm and 150mm is not mega large but will be noticeable (not to speak from the better resolution capabilities of the larger instrument if seeing permitting).
The Maks often have had standard coatings instead of enhanced coatings. At least that was the case six years ago. The enhanced coatings have s little more light scatter apparently and may not be considered as good for planets, and many people get Maks for planets, so it seems different coatings were often employed.