Hi,
First, the HikaShop import process handles only CSVs. And that much products in an import file will be a problem for it. The import has been designed for flexibility and thus it's not adapted to handle that many products. A custom built import will be better in that case.
We've seen HikaShop with a few millions products on some of our users website, so 300 000 products should be ok.
The problem is more about have a server powerful enough to be able to handle the load.
I don't remember the URLs of these websites, and we don't keep a list of these on hand, and we don't know what users do with HikaShop unless we have access to the website for some debugging. So I can't give you any link.