I run a M Gardevoir myself and I play test it at my league. I run into very little issue as long as my Megas aren't prized. Your best bet: lose the Xerneas Break. It's too cluttered. To replace, put another Giratina because it really helps with those match ups against megas. Also replace Hoopa...
First time I attended a prerelease with my girlfriend was Steam Siege. We both pulled full art sycamores and our friend pulled a volcanion ex. It was pretty sick to be honest. We made our money back obviously but I can see where some people don't make out with it.