It probably depends a lot upon whether there are any customizations deployed on sales orders already (workflows, user event scripts, client scripts), especially if those customizations do any looping through or processing of the line items. The only way to know is to test it out. The APM bundle can help pinpoint performance issues with record types and pages if you find them.

Sure, I was thinking if they had zero to no scripts processing line items, does the UI keel over at 1k or 10k lines ... but someone mentioned in the thread above of their installation with 10k lines so that gives me some hope.