Design letters

Once a design gets to 100% is there any advantage in continuing to develop that part?

Or to put it another way, Is a 10F better than a 10E if a part is already 100% or are the letters irrelevant and it's purely the % which matters?

The % is what matters. If a 10F Part has 80% but someone else has 7A Part with 80%, it is the same performance.

You can't get better than 100%.

What's the next letter after one reaches 10Z?  10ZA?  10Z1?  10A1?  10AA?

what is the next  letter after 5Z

Pretty sure I've read somewhere once that it will stay Z but you will still get extra performance for each update.

yes stays Z