Weight limit is a much bigger deal than you appreciate, properly supported and cautiously played, many characters can survive in WoE until their supplies run out. Certainly there are instances where people get caught out and focused down, or lex fisted, or support simply plays too poorly to help people survive, but that doesn't stop someone who's using slim whites from having a significant advantage over someone who is using mastelas.
In a fast paced game like RO, where living and dying is a split second difference, tiny advantages often make all the difference. It's why guilds with similar numbers of players can sometimes have overwhelmingly different strengths as a group, and 10% is also a bigger gap than it sounds like on paper. Combined with other small advantages, they add up to be a big deal.
I also don't understand why it is even a goal to make mastelas a bad item. I don't think that having relatively easy access to a fairly comparable alternative is a bad thing. Someone who doesn't have the time or inclination to go the extra mile to pot slim whites should still have options available. Mastelas, while not optimal, are still close enough to be reasonable, you don't just automatically lose for using them, but someone who has made whites is certainly at an advantage. If, for some reason, you really wanted mastelas to just be gone, simply removing them from the game would be a better (but still bad) decision than changing white potions, as it would at least not effect game balance, but just make it more work to compete.
In short, the balance of healing items is not something that is broken. It is not an aspect of RO's gameplay that needs adjusting. It is not something players are currently unhappy with. There are plenty of things you could look at changing that people actually are unhappy with, I don't see the need to break game balance over something that everyone is already okay with.
Also, I've said it before, but apparently it isn't coming across correctly: you can't raise effective healing rates by almost 50% and expect game balance to just be okay. FIFTY PERCENT. Imagine it from the other direction, buff the damage of stormgust (the strongest AoE) acid bomb (the strongest ranged) and sonic blow (the strongest melee) by 50% to compensate for 50% increased healing. Is that a reasonable change? Would anyone look at those numbers and think, "oh yeah, that'll be fine, lets try that out for a WoE and see how it goes?"