I think anyone who is interested in a lisp has been positively influenced by Paul Graham, I certainly have.
You said this in the HN thread:
"I'm sure Arc is the wrong choice if I want to be as effective as possible in 6 months. If Arc is only the best choice if I want to be as effective as possible in 5+ years someone save me ;)"
The specific language is only one aspect of effectiveness - others include available libraries, an active contributing community, etc. I don't think there's reliable evidence that Arc will change enough in 3 years to fulfill your "effective as possible" goal.
Paul's last submission to his own forum has been over 5 years ago! And his last comment has been over 4 years ago. It would seem that he's not actively involved any more.
I was very excited to hear about Arc over 7 years ago, and I had high hopes that Paul might be the person to create a successful lisp and rally a great community. This has not happened, and I think it would be foolish to think this will change much in the next three years.
In the end, I ended up choosing Racket as my lisp - it made my short list to evaluate in part because Paul chose it to implement Arc :)
I'm certainly not trying to "save [you]", but I see many similarities between your quest for the most effective language and my own and countless others :) The language is very important, and concision is important, but you really need to evaluate the entire ecosystem to make a good choice.