Here's my opinion on the whole thing.
Imagine you buy an AAA game at $60. you enjoy said game, and you finish said game. They then release a cosmetics pack for $120. Would you buy that? I figure you'd say no. It's practically the same here, Minecraft is a $26 game. As said above, the E+ rank is $140. Which is over five times the original amount of the game! Let's go back to the $60 game. If EA had released their cosmetic DLC pack at $120, I doubt anybody would buy it.
For a server that's trying to desperately raise its player count, I feel you should really listen to your community more. If you think about it bare-bones, E+ is just some fancy colours, more building space and flying. Oh and of course, bragging rights.
If you follow the current curve of prices for ranks, which is;
VIP - $20
VIP+- $40
ELITE- $80
E+ -$140
the prices doubles, doubles, and then 1.7x. If we're going to do $140 * 1.7, that would bring the price to a whopping $240. May I remind you that the original price of Minecraft is $26, meaning this rank will now be over nine times the original price of the game.
If the perks of Elite++ are anything like Elite+ (Which are, extremely lacklustre) then I would not recommend buying the rank. However, we don't know what the perks of the rank will be. May I remind you that due to the EULA ( End User Lisence Agreement, in Minecraft's case this essentially means that you can't pay to get gameplay boosts/benefits. E.g, if you were an E+ you would gain double emeralds in Murder. This would break EULA.), the perks will be COSMETIC only. Meaning they will only give your avatar or name visual perks. This would be something such as changing the colour of the + in E+, or giving your character a glow. Neither of these give gameplay benefits.
This is all I have to say to the subject, and if you're buying a rank purely to support the growth of the server, good on you! But if you're like the rest of us, and would like some perks in return, then this rank may not be for you.
Thanks for reading what I had to say.