They're a US-based youtube channel that started by posting builds, guides and tutorials showcasing their work on customer guns, eventually moving onto developing/adding input for custom parts to have made [ala negative helping to spec out and design parts] and from the outside looking in they've got good results. This is no different to the idea of Bolster armoury stuff, high end prebuilts that have warranties so if money's no object and you just want """"the best"""" or whatever I don't see why it's such a bad thing, even if it's pretty tasteless imo (the black topographic markings are cool though)
The limits of 400/350 are probably for outdoor/indoor limts in the US. From what I'm guessing these use a CYMA MK18 platinum (or whatever it's called in the US, a EMG? idk lol) as a base gun with the topgraphic markings and receiver cuts milled/cnc cut into the receiver alongside the drop stock and other maxx parts added.
Them being US based is a bit of a nightmare though as I've no idea if they've got somebody this side of the pond to do repair work or maintenance, and the DSG linked in the OP will need looking at every now and again because it's a 40 RPS DSG at the end of the day, or if they'll even ship to the UK regardless with all the hoops you've got to jump through to do so.
Like everybody else has said in the replies - you can get in touch with them and let them know what weight & joules you'll be wanting to use and they'll tune the thing to that spec.