I really like the MASC on the Osiris A. It's not something you want to fire all of the time by any means, but given the amount of times it allows me to bug out in a hurry, sprint away from approaching Harassers, dodge missiles down a hill, etc. it's well worth it. While it makes a lot of heat, it's not something you want to use while on the offensive all of the time anyway - it's purely an "escape and dodge" tool and in that aspect it works perfectly.
In short if you're using it all the time to the point you're having steady heat problems, you're using it wrong.
Note: For those of you bringing up CBT into this, I'd like to point out there MASC has a huge chance of outright
locking your mech up completely and leaving it worse-than-legged on the battlefield. That is, every single time you use it, it can fail and wreck your mech - and the more you use it, the worse the chance. The point to this is it's something to be used
sparingly for emergencies in it's original intent, to get behind a unit that you REALLY need to or to escape a bad situation. In that regard, the high heat build-up keeps it very much in-spirit with the original intent.
Just to make it clear I do by no means try to fit the Osiris into a specialized min/max role. I just have the feeling that the Osiris A's current load-out is impractical for most players (I actually think that the current load-out is far too specialized - a change would make it a easier chassis in a lot of different situations imo). Also I don't want to enforce a loadout change by hard, I want to share that thought and discuss it. If it turns out during discussion that my observation is wrong, then that's it 
That said, there's tons of tweaks and changes I'd make if customization was in to a number of variants, outside of just making my own. Right now without it, "accepting the negative aspects to your style" is just part of selection, unfortunately. I do think there could be more variants to represent everyone's tastes - in particular in the Medium-Heavy categories - but I can't think of a single design that I wouldn't tweak something on if given the option to make it more effective.
All I'm saying is you're not wrong, possibly, for you. I like the variant with the MASC, but even I'd consider stripping it off and adding a few more tons of armor if the option was available to me. But without customization I don't really think it's necessary to make each design as "effective as possible" but just "worth the money you pay for it." Osiris A fits that right now.