Never use a regular wall plug for anything other than 120 volt 60 hz!!! So it does violate the NEC coding. I would recommend those disconnects I showed you sold in Radio Shack or electrical aisle of hardware store......Good job for grounding, bot for extra protection I would add a green wire with 2 ring connectors to put one on where the current grounding wire is and other directly on one of the holes of the ballast screwed in just for extra protection!
This ballast does not have an ignitor, plus you have to check the voltage rating, I usually see 600 volts....also in HPS ballasts connected to socket setups with ignitors I have seen those quick connectors used...you know those with flat terminals.....
Neautrik makes a Coaxial Power Jack that might suit a Ballast Box...the connector is similar to a Coaxial DC Jack but larger...and Meant for ac...it is Rated for 250 Volts at 20 Amps.