Ouch...
I would definitely be careful with that shop...
Your fly wheel doesn't do anything but give the starter something to turn in order to start you engine - and with an automatic transmission it is actually a flex-plate (fly wheel is for standard transmissions)...
A torque converter - to my knowledge (and, I am not an expert in this) will pretty much work or not... theoretically, it could be the culprit for loss of power - but, usually something will break in the torque converter when they go bad - and that, you will definitely know when it happens...
Are you experiencing any loss of power on acceleration or hill climbing...?
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