Btw! It could also be further improved to only add real order number on success, and also instead save the temp orders to it's own database table etc.

Personally I don't see this as too much of an issue, the generate invoice id allows for sequential invoice ids (which are required for VAT in the U.K).

Some OC clients have requested that order ids were randomly generated or order references used to disguise their sales figures. If this these changes are implemented perhaps a setting could be created that would allow a store owner to chose how order ids are displayed to customers too.

Having 5-10 temp orders stored per customer session is generating quite much unneeded data during the years, surely it can be a quite big issue in the long run. But maybe you was only talking about order id's, which isn't directly related really - order id's and invoice numbers is different things.

Btw! The temp order should probably only be saved when the customer has started a checkout, meaning there is at least a name and address on it, else it's more or less useless anyway.