hmmmm this doesn't convince me 
You will need to add extra code anyway so a good option is test the FM with the flag and then comment the FM and use the BOL query. I'm saying this because as understand the CRM product information shouldn't be buffered in the BOL so even using the query or a relationship the system will need to access to the DB, probably the genil layer will call the same FM which you are using or similar, that's my assumtion. Having the FM parameter unchecked will lead you to a direct access to the DB each call, even if the product was used in a previous order/line so I guess this is what gave you those performance problems.
Once you finish this test will be cool to share the results/impressions with us 