Having a higher level workshop does not give higher chance for epic modifications. I have a level 16 workshop and that just gives stronger modifications instead of stronger modifications with higher change for epic modifications.
I currently have the bought Phantom Epic outfit. I have 3 epic modifications in it and 2 rare modifications. I spent donuts re-rolling for the epic modifications I have in my suit. Only 1 came naturally from refreshing daily. I bought my suit at level 310.
I wanted to buy a new suit at level 350 but the only Epic modification I have gotten in the last while is for the boots/shoes. Meaning if I buy a new suit it will be much weaker than the suit I have on. I am currently level 355 and still waiting for replacement modifications.
There is no point buying a suit and waiting for the modifications to be made as I could get a stronger suit if I buy it later when I can fill it properly at my then current level.
Even if I raise my workshop to level 17,18,19 or more there is no better chance to get the items I need. At this rate I think I will be closer to level 400 than to 350 by the time I get the modifications I need.