Time and time again when people complain about pitch and setup issue is low tunings is almost 99.9% pick technique. I have known guys who use a 70 or even up to 80 in drop B and still have pitch issues, most of them always shocked to find that I was using 11-54 in drop B and had no pitch issues.
I moved up to building my own custom set of stainless steel 12-60 for a tad bit more stability and a more girthy tone because my guitar sounds a little weak especially on the plain strings for clean stuff and I wanted something more even in tension on the bottom to compliment. Spent hours doing mass and tension calculations to get the perfect set. I can't hit the strings hard enough to go out of pitch, at least with the way that I pick. I could pick like an idiot and make it go out of pitch but defeats the purpose. I also play with a floyd and they love to go really flat on all the other strings when you hit another one too hard and if I don't have problems using heavy ass jazz III picks...yeah its all in the pick hand.
A 25.5 inch scale PRS can do drop B if setup properly, any 25.5 in scale can, the only factor in the guitar is how it sounds as a tonewood and how it feels. If it feels good and sounds good it can do just about any reasonable tuning.
Absolutely would never touch an Eclipse or any short scale guitar for anything under drop Db. I don't like them in any tunning, feels way to cramped, baritones feel way to spacious, cramp the hands and even with equivalent string tension the strings still feel too stiff, even when the strings are flopping like crazy, still can't bend for shit.
work on your pick hand, try a set of 12-60 or something similar. If you don't want to go custom try out the GHS 10-60 boomers, or the DR DDT-12. My custom set is D'Addario Pro Steel 12-16-20p-30-40-60, the 60 is a bass string and the plain are just regular plain. Costs about $20 with shipping but is completely worth it, they last much longer, much longer than crummy nickel and they sound better. Get something heavy, work on your pick technique, get your guitar setup by someone who knows how to properly set up a guitar for low tuned metal.
Something to also keep in mind I find people that have their action too high are more likely to have bad picking, as if you hit the string too hard you won't get much fret buzz however if you have low action and your picking is bad that would cause sharp attack in the notes, you will have severe fret buzz and it makes you more aware of your technique. High action also causes intonation to go out the window because the extra tension you have to apply stretching the string causes it to go sharp and since the ratio of tension to pitch is not linear relative to string mass, even though two strings were stretched equally from fretting the same fret, one fret will be more sharp or more flat than the other, and even if they are in pitch with each other, chances are, they aren't in pitch with your 12th fret harmonic or open fret. If you have to press down too hard because of high action over pressing the string against the fret not only ruins the fret lowering fret life but also causes you to stretch the string more against the space from the fret and the fretboard, more pitch drift over time. It also doesn't help that you first fret the string harder when you first play the note and let off as the chord or note sustains, furthering the extreme between a sharp attack and a flattening decay.
So repeat after me: Setup and technique, setup and technique. Rinse and repeat until it registers.
