Forgive me if this question has been asked before, I did a search and did not find it.  I have cut up a chuck roast into steak slices (relatively thin, maybe half an inch), tenderized it and marinated it overnight. I am now ready to grill it. I have read contradictory advice online on the best way to grill it, the disagreement being whether to grill it 'low and slow' or 'high and fast' to ensure maximum tenderness (that is, to preserve whatever tenderness was there and ensure it doesnt become 'tougher').  I know, chuck roast is not typically designed for grilling or doing what I'm doing, so I'm not looking on a lecture on that, I'm simply asking if I want to preserve whatever tenderness is there, will I have a better shot grilling it low and slow or high and fast?   As a side note, its grass fed beef, and just by throwing a few small pieces on a frying pan last night for a few minutes yielded (surprisingly) relatively tender meat.   
I know and have read over and over the "correct" method of cooking this type of roast is low and slow (meaning for hours on end) not sliced up and on a grill like I did. Its not a traditional steak cut and its not 'supposed to' be treated that way. If you are looking for the best way to cook this meat, follow all the advice you find online and in the other answers/suggestions here. However, I chose to go against that advice and slice up a chuck roast, tenderize it and marinate it to see if I could cook it like a steak. My results were that you can, and the results were surprisingly good. Your results will vary on the quality of your cut and how you prepare it, but it CAN be done effectively.  Because I had beat and cut the chuck into very thin slices, putting them on the grill on low heat for more then 10 minutes was enough to overcook them. Thankfully I only did a few like that and the rest I grilled like a regular steak for a few minutes a side and they turned out better. I don't believe this had to do particularly with the cut of meat, just the fact that they were so thin already. 