This meta looks pretty cool, but I don't think you should've just given the same stat raises that the real mega gets, even if you halved the raise. You probably should have ignored the 50 BST raise for some mons, such as Magikarp (go higher, come on), Gastly, and Meditite (too much of a raise). The way I think you should've done it is slightly hard to explain, so just let me know if it doesn't make sense.
The way I see it, in OU, base 100 stats are "average." It's good, but it can be a LOT better, and usually you need a good typing / movepool / etc to succeed with stats just around that benchmark. I see the LC equivalent of that being around base 60. Usually if a stat hits that benchmark or is higher than it, it is considered good (although higher is preferred). Now let's look at how mega-Gengar and mega-gastly compare to this benchmark. Mega-Gengar is 70 points higher than the OU benchmark, whereas mega-Gastly is 60 points higher, double that of the benchmark. This is absurd. Just keep it at the original base stat.
Another thing you need to keep in mind is how the mega evolution changes the pokemon. For instance, Medicham starts off as a strong, slow, and really frail wall breaker, and turns into a really strong, reasonably fast, and still quite a frail wallbreaker. The big thing here is the increase in speed. Meditite goes from a strong, kinda fast, kinda frail mon into a fast, really strong, and pretty bulky mon. I don't think this is right, and as it stands, Meditite is nearly broken in regular LC (Gligar getting banned will make it broken). I would make the new mega spread 30 / 50 / 50 / 40 / 50 / 70. This should make it more balanced, and the big thing here is the enhance in speed. I am ignoring the fact it is bulky with Eviolite since I don't think you should have that with their mega stones. I'll get to that further in my post.
Some of the pokemon have really poor stats in relation to the meta, and in relation to how their evos compare to the OU meta. Ralts is one in particular. With your spread it has poor physical stats and HP, average SAtk, meh SDef, and eh Speed. Mega-Gardevoir has poor physical stats and HP, amazing SAtk, great SDef, and good speed. That really doesn't match up at all. This is due in part to Ralts' already bad stats, but the mega evolution's purpose is to highly improve a pokemon. Try something similar to 28 / 35 / 35 / 95 / 65 / 60. The SAtk may seem really high, and it may look a little broken, but stuff that has stats like that (Gligar, Yanma, Tangela) have great stats all around, not just in one spot.
Magikarp, oh god Magikarp.... Why did you even bother with this one Just make the new stats comparable to how Mega Gyarados performs in OU. Something like 20 / 85 / 60 / 20 / 65 / 70. This gives it good attack, nice bulk, and nice speed. I dropped the speed since Gyarados isn't that fast, and 80 is quite fast for lc. Also give it Crunch (or bite if you feel it should have that instead), Stone Edge, and Taunt. These are moves that are generally staples on regular / mega Gyarados. That should be enough to make Mega-Magikarp viable.
This part is huge. Please, please, please get rid of the Eviolite boost from mega stones. This makes things that do get nicely improved bulk from their lc megas have even more bulk than their fully evolved counterpart would (i.e. Meditite and Gastly). On megas that are meant to be much bulkier than their normal forme, just raise the bulk so that it would be comparable to their Eviolite boosted bulk with your current spreads. Otherwise you'd have stuff like Meditite actually taking a +2 Shadow Ball from Missy and koing with Zen Headbutt, something it can't even do with it's normal forme.
All that being said, basically what you need to do is put some time and care into these mega evolutions to make sure that they are viable and not broken, and impact them similarly to how their evolved formes are impacted. Please don't just disregard this post. I may be new to the forums, but when I speak up and say something, it's usually something important and worth listening to. And if you find you don't have the time to go through and fix all the mega evolutions (not ALL of them need to be fixed, Riolu, Snover, Shuppet, Houndour, and Charmander X look fine to me), I can go ahead and do it for you. In fact, I would like to collaborate on them sometime.
tl;dr: just read it