Babgrip
I sold all my Altec horns and drivers on Ebay once I found the D210Ti did the job as well or better to my ear, and it looks WAY better on a spectrum analyzer... fairly flat now out to 20khz whereas the Altec stuff dropped off around 12khz. 'Coarse the D210Ti can't do 500 hz, but - as you said - you can buy a pair of them for the same price as a replacement Altec diaphram. The D220Ti is suppose to be everything the D210Ti is, and more, for the same price. I have not bought one (yet?) so I don't know for sure if that's true or not.
I've never owned any Altec woofers. I used JBL D130's and JBL D140's in my first set (D140's sounded better), and I'm running JBL 2220's now. My only problem with metal dust caps occurred when one got dented. It would resonate at a certain frequency (depending on the size and placement of the dent I'm sure) such that when that note would hit in a song it sounded horrible. I cut that dust cap out and the problem went away. I don't know what removing or replacing a non-dented metal dust cap would do to the sound...I would guess not much.
If your dust cap is dented, then you have probably thought about protecting it some how. Years ago I came up with an easy solution. You take the woofers out, staple in an 18" square piece of speaker cloth, and then reinstall the woofer. Only problem is that you can't see that cool shiny dust cap anymore.
Dan