Here are my thoughts, just out of interest...

(1) I am not sure why those columns are mediumblob anyway. This seems like a really poor choice. (So, if someone rewrites they could consider this)

(2) Anyway, I don't think (but may be wrong) that all that data that is stored in blob format is ever used. I think it is just record keeping, of the state when the payment was processed, which is a good thing in some circumstances and may be a compliance issue .... maybe possibly.

(3) If it is decided that you need to store that information then it could be stored in a different table or even dumped to log files. Log files might be a nice solution and then the feature could be turned on/off as is done for some other payment modules logging.

(4) There is not a huge amount to recommend Worldpay as a web payment gateway. The interface and communication are just plain nasty in my opinion. I understand that the POS stuff works better and Worldpay is cheap, but I would really consider another option.

It is (4) that is the kicker for me. I got involved in trying to understand and streamline the module and ran away screaming. Happy to give some advice/opinions but I do not want to get into the nitty-gritty of this module. I value my sanity to highly :-)