Happy to do that bluskis.
I'm actually thinking of the cost-benefit issues for a Mode S Elementary setup - in other words you pull out your Mode A/C transponder and stick in a Mode S one, connected to nothing more than an encoding altimeter. No FMS required. That's what's proposed for aircraft < 5700 kg in the TMA for 2005.
The purported advantages of this rest on two issues, both related to capacity.
1) There are a limited number of Mode A codes available
2) There's congestion at SSR RF frequencies when Mode A/C is used.
I'd like to understand whether these are real issues, now, and if not, when are they likely to be serious limits to capacity.