Yes, they will mostly like dominate. DCE exists in most formats so you can't take that away no matter what. I'm juts going to assume Catcher is a Supporter not an Item. Even then, the amount of damage that could be dished out is pretty overwhelming. In an XY format, EX's don't seem half bad because the whole format has been made around it.
About DCE, not really. After its reprint in Base Set 2, DCE was absent from then on all the way to HGSS, making it absent for 3 whole Generations. In other words, it was out of Standard from 2001 all the way to 2010. Nearly a decade, or about half the age (more than that, if I calculated correctly) of the TCG, hardly most formats. To date, only 5 out of 15 formats had DCE in them, and for good reason too. It is in fact a source of single card +1 energy acceleration with no drawbacks at all, amd I can name no other card that does that. To no surprise, most of the more competitive cards in the format, like Mewtwo EX, Toad, M Ray, Raichu, Night March, Flareon, Donphan, Gengar, Trevenant etc. relied on it really heavily. It severely breaks pacing, from the very moment it entered the format when Garchomp C Lvl.X became able to attack for only a single Energy attachment and snipe out just about anything.
As for whether it would break the format, honestly it is hard to tell fully based on theory, but long story short, it is all about implementation.
Whenever a new mechanic is introduced, they tend to end up unstable for the most part because the newer mechanics could make use of support from prior sets just as if not more readily than the ones that have been there before, which is indeed the case for Pokemon-EX. Their older relatives, basic Pokemon-ex couldn't make much of a splash back then because they didn't get nearly as much support with so many following Trainer cards specifically going (excluding Pokemon ex), and were far too weak to justify their use. Pokemon EX had cards like Skyarrow Bridge, Dual Ball and Pokemon Collector effectively giving them much more support than they should be getting, since the game was attempting to transition from Basics being weak in general, to all fully evolved forms being able to stand side by side. They also had DCE, which Pokemon exs did not have, and attacks that were overly cheap (common among early EXs) combined with late game power.
XY on Basic EXs were much, much more stable than they are having either early or late, but never both. The only exceptions were Lucario EX, Florges EX, Yveltal EX, Manectric EX and Rayquaza EX, while everything else had little to no early presence.
Personally, if Basic EXs were called, mechanics-wise, 'Basic EX' instead of just 'Basic', they wouldn't be as overwhelming as well. They would improve the game from being either all about evolutions (pre-BW era) or all about basics (Base Set era) to something in between, but only if they had been implemented better. If HGSS hadn't went back to the 2x Weakness format, then there would have been another opportunity there as well, since you could have made Basic-Stage 1 - Stage 2s +10/20/30 respectively, and EXs x2 so they were more easily countered than their non-EX counterparts. This would have made, for starters, countering Pyroar into more than just splashing Beartic into any deck that ran DCE.
Regarding damage, the biggest problem Basic EXs had back in the NXD era was that they did too much damage too quickly, and had the opportunity to do said damage off the bat. If Mewtwo EX was instead Lugia EX and had to wait until turn two to hit Aero Ball due to no DCE, or Zekrom EX needed 3 Lightning Energies instead of Lightning + DCE, or Landorus EX couldn't attack first turn due to rulling changes, Pluspower wasn't around, nor Catcher, nor was weakness for non-EXs multiplicative, nor did cards that benefit Basics chose Basic EXs as valid targets... In order words, better implementation that aims to give Evolutions a little more time, then Basic EXs wouldn't have broken the format as hard as they did.
