I can notice a difference with 2mm. If you dont feel right still then you need more and then you are into changing parts. Remember you can use combinations of things like knee savers and ti speedplay. Knee savers are actually +19mm ti speedplay are -5mm overmost pedals. Thats all up +14mm but you currently have your cleats wide. Speedplay cleats set narrow could take off another 5 or 6 mm getting you to +8mm You say you are too narrow as it is and want to try an extra 2mm so knee savers could be used to get just an extra 6mm fram what you are about to try with washers.

Just a thought if you cant find a slution and decide you need to spend some money to get it sorted. There are of course any number of combinations of things out there that cost varying amounts

BTW get the washers from your LBS they doint cost much and will ensure you get hte right size