0) I don't know of anyone who has done that in that particular manner. That doesn't mean that it cannot be done.
1) The performance considerations would be all of the usual suspects. Good storage area design, up to date release of Progress, 64 bit OS & OE, lots of RAM, excellent disk (think SSD...); plus, since this will be accessed with SQL, a really, really good network, routine use of UPDATE STATISTICS, monitoring of common queries to determine if new indexes are needed and so forth.
2) Licensing? Yuck. If you can convince yourself that this is a "bolt-on" to an existing app then maybe there is no licensing cost. Otherwise you're building a 300 user system. Then you need to figure out if those are 300 "named users" or "client access" users or what the heck they are.
Whatever you do, do NOT say "300" where a Progress sales person might hear you! That will immediately become the lowest number of the highest priced option that you will ever see a quote for. The first thing the sales person will do after listening to your voice mail (they never actually answer the phone) will be to forecast the revenue for the current quarter while heading out on to the golf course with his boss to celebrate. (His boss will then increase his quota for the next quarter...) The next morning, after a hard night celebrating, the sales rep will have an assistant put together a quote that starts with whatever he thinks he remembers you said plus several feverish fantasies that will result in a new car, an upgrade to his yacht and, with any luck, a fresh lakeside mansion. The boss will check in over lunch (probably from the golf course where he is celebrating his good fortune with his boss...) and want to know when the deal will be closed.
Start by telling them that you think you may launch with 10 and build up towards 20 users and ask what your options are. Then ask what sorts of discounts you might be able to get in the unlikely event that you can convince a total of 30 users that the system is any good... you might want to mention the down economy and any recent downsizing or threats thereof (anywhere in your own company is best but competitors are also a good source of bad news).