I believe you are missing the point. It doesn't matter if it's 50% or 75% or 99% humidity at the intake. It will not change the amount of work the stack has to do. Please allow me to show you.
Imagine a really big, really squishy, really absorbent sponge. Like one of those beige car-washing sponges. Now imagine that it is filled with 50% of its capacity of water. For even numbers, let's say it can hold a gallon, and we've poured a half-gallon into it. No problem, it's all in the sponge.
Now squeeze the sponge to half its volume. At this point, no water will have leaked out. If the sponge had been at 100% of its capacity, then 50% of the water would have been squeezed out. If at 75%, then 25% (of capacity: 33% of what we put in) would have been squeezed out. But we were at 50% so we squeezed out no water. But the sponge is *now* at 100% capacity, because we squeezed the sponge and reduced its capacity.
Now, when we squeeze it further, we will get water out, no matter were we started: 50%, 75% or 100. In addition, from here on out, we will get *exactly* the same (additional) amount of water out of that sponge. That's because no matter where we started from, at this point the sponge is 100% full. It doesn't matter *how* we get to 100%, once we get there we get the same amount of water out of it.
The sponge is the air. The water is the sponge is the humidity in the air. Squeezing the sponge is compressing the air. The difference is, the compressor isn't compressing the air to merely 1/2 its volume. If we're compressing to 1500PSI, the air is now 1/100 the original volume. Even if the air were at 1% humidity in the beginning (and it's *never* that dry, even in the middle of winter when your furnace roasts freezing cold air to 75C or more..), it would *still* be at 100% humidity at that point.
So whether you started with 50% humidity or 75% humidity or 100% humidity, you will *end* up with 100% humidity no matter what. The fact that you're getting water from your drains proves this. Where do you think it came from? It's the water that the compressor is wringing from the air-sponge. If we're draining water we're at 100% humidity at that point. If we're not at 100%, we're not getting water out from a *mechanical* process like our drains. Again, no matter what the incoming humidity *was*, if you've got water in your drains, you've got 100% humid air going into your filter stack. It's physics!
The bigger factor is *temperature*. The "humidity" measurement is *relative* humidity. The *absolute* *capacity* of air to hold water (that is, how much water by mass is in the air) increases with temperature. So if you have 50% humid air at 0C and heat the air to 75C (like in the furnace example I gave above), you'll drop the *relative* humidity dramatically. But same mass of air. Same in reverse. If we have 50% humidity in warm air, it will contain *more* water by mass than 50% humidity in cool air. Same is true at 100%: warmer air will have more air by mass than cooler air, even though they're both at 100% *relative* humidity.
So, forget intake humidity. You're going to exceed 100% humidity no matter what you do: your PMV *guarantees* that -- that is its entire *JOB*. You will get more water out of the drains, but that's *their* job, and you don't have to replace them like filters, so who cares. *Temperature*, though, *does* matter. If the air entering your compressor is warmer, it'll be even warmer after compressing. If the air is 10C warmer going in, it might actually mean that the air coming out is 20C warmer -- because cooling will be less effective, too. And because the *absolute* amount of water in 100% humid air is higher when the air is 20C higher (even though they're both 100% *relative* humidity), now your filter stack has to work harder.
ETA: By the way, I am too lazy to look up a relative humidity curve. It very well may not be linear for either temperature or pressure like my example sponge math may imply. But it's irrelevant: the point is, once the air is at 100% it doesn't matter how dry it was *before*, it's going to be the same no matter what. It in no way changes anything about my example: this is just to address the ScubaBoard lawyers that want to nit pick everything...