But the financial costs of these to UK corporations must surely be less than the financial/economic benefits of being part of the EU, mustn't they?

The Department for Business, Enterprise and Regulatory Reform has a web page titled What if we Left the European Union? which lists downsides such as:

• we might lose the advantages that economies of scale bring to pan-European industries such as  car manufacturing or aerospace;
• we would have to bear the costs of renegotiating bilateral trade agreements
• we would risk losing direct inward investment from companies which see the UK as a gateway to the EU.
• If we wished to continue trading with the EU - for example as a member of European Free Trade Area (EFTA ) (like Switzerland) or the European Economic Area (EEA) (like Norway) - we would still have to comply with EU laws, while having no say in negotiating them.  We might even have to keep up contributions to the EU budget as the price of continued access to the Single Market, but get nothing in return.

Are the above points are just pro-EU spin that just try to cover up the larger downsides (to corporations) that you listed above?

Truth unfolds in time through a communal process.