Fiber COULD be 'better' than your coax cable connection... but if they sell a lot of service in your area, it can and probably will feel congested at some point... upstream of you all those optical cables get concatenated into fewer pipes. Then THOSE fewer pipes get bundled at HQ or substation or whatever, and the signal at some point passes to whomeever is supplying ATT with their internet backbone connection.
This is true whether you use fiber, cooax, or anything else. At some point you WILL share bandwidth.
50 mbps is pretty good. If you're getting real speeds of 5-7 MB/sec for downloads you're getting excellent results for it... I'm assuming that's what you meant even though you actually put Mbps. Correct me if you actually meants Mbps.
No comment on the TV side. In terms of noticing download speed by dropping to 18mbps... yeah, you'll probably notice it when you hit youtube, patch a game, etc. Will it be crushingly slow? Youtube, not so much. You generally only stream one of those at a time, and 18mbps is fast enough. Game patches you'd notice doubling the time to patch.
All speeds are theoretical... fiber, copper, wireless, what have you. In theory you might get more of your 18mbps fiber than you are of your 50mbs copper... but 80% of 18 is... 14.4 mbps... and you're currently getting '32 mbps from Speedtest. And no theory involved, 32 beats 14.4 twice over, with some to spare.
I'd also like to point out, that if a bunch of people in your area ditch Comcast for ATT... you'll have all that coax to yourself, soon, at zero cost
Have you noticed traffic congestion on your cable?
Remember, salespeople are salespeople. They're programmed to sell. The truth is sometimes an inconvenience to them.