I've already got code to display the additional info in product info, product listing, basket, checkout, historic orders and in the admin (from another shop) but this was all based on storing the extra fields in the product table not in an 'extra stuff' table- so rather than doing a lot of re-writes I thought I'd go this route.

Anyhow I've decided to make copies of the two tables and try the merge. I'll export the data before and after into excel and do a compare - then when I'm sure it's woking like I want I'll do it for real.