you'll get some stunting, but it's nothing like brassicas. some brassicas are the worst. It's really just wasting seed that's the primary cost.
Heck, look at what Sturgis posted over on the other forum. If I recall, he's planting 250lbs of oats (50) and rye (100 + 100) he's planting per acre.
On a side note, don't waste your time planting 100 lbs of cereal rye, only to return two weeks later to plant another 100 lbs of cereal rye, so that "reinforcements are ready to take their place as the first planting of cereal rye is eaten" (paraphrasing, but that was his point). Cereal rye doesn't stop growing when it's eaten. It continues to grow, just like grass in your yard. If you want to plant 200 lbs of cereal rye, plant it all at once. There is no tangible advantage to doing it twice.
P.S. Planting stuff like peas first, returning to top seed cereal rye after they've germinated is a good move. It gives the peas a head start...planting 1/2 the cereal rye you want to return later and plant the other 1/2 is what I was referring to that makes no sense at all.