Not having one Regirock in this deck is a lot like not having a Claydol in most decks. Especially when you're playing Stark Mountain as your counter-stadium, you might as well get an extra attach every turn when you need it.
I think the difference between a Groudon-only, and a Groudon+assist deck is that the Groudon only deck can't ever attack on turns 1 or 4 whereas Groudon + assist can attack every single turn (including, theoretically the 1st, although the odds are very bad) and simultaneously build another threat while Groudon #1 is wreaking havoc.
Edit:
After some testing in RedShark, I see that Mulligans are going to be a serious problem for this deck, even in my version that includes Regirock.
In 30 games I opened, I had 19 mulligans, 7 'Good' starts with a Groudon and enough energy (including one setup for a turn-1 attack), 1 Regirock-only start, and 3 starts with fewer than 3 fighting energy (though in each situation, I had a Roseanne's research.
So given my deck in the above post, for each 'Good Start' you're going to be figuring in about 2 or 3 extra cards for your opponent to work with, and about a third of the time you do get a start, you'll have a problem. You can compare this to the 4/56 deck which tends to have around 4 mulligans for every good start, but which is 100% consistent on a T2 kill, given no intervention from your opponent. So I may have to admit that a less trainer-heavy and more energy-heavy deck will be the solution.
Though I will still adamantly argue that too many Pluspowers/Bucks may be as much a problem as anything else.
I'll mod the deck a bit and see what I can do.
Helghast your new list looks pretty good, but I'd probably take out a buck's or two, a felicity's or two, a pluspower maybe, put in more warp points for disruption, probably put in Energy Restore over TSD just b/c its cheaper, maybe add a night maintanance, and take out a quick ball or two. More Roseanne's and Stark Mountain (Definitely 4x) are obvious ways to get energies (especially if you're playing regirock) or kill enemy stadiums like Snowpoint that might deny you OHKOs.
I was also thinking Rowan's might be a good card for this deck, allowing you to keep something useful and get a whole new hand of 4 cards without having to discard anything.