I think it won't improve your yield much Fred but iron is recommended as a trace element for all crops. If your soil is extremely alkaline then it will certainly help and, used sparingly, won't do harm in any case.

I've never known any onion growers wanting to use it.
Why do you want to use it?

I thought onions liked alkaline conditions?

They do Bamboo. Or rather they are not at all fussy unless the pH is very high or very low. In the former case Ferrous sulphate will help but so would a bucket of peat. A much more useful sulphate to add would be potash.