Exhaust - dump pipe, probably best off waiting until you replace the turbo with a highflow They have a tendency to go after 100,000km's, in Japan I believe they replace them during that service. It's not all bad though as a highflow pulls like a freight train and is going to last a lot longer due to getting rid of the ceramic turbine. Besides with the stock turbo I doubt you are going to find remarkable performance upgrades from just an upgraded dump pipe alone.
ECU, so far in the country that we know of the Nismo & Impul aftermarket ecu upgrades have been tried and tested in the AR-X. From the opinions and stats that have been provided the Nismo seems like the choice of the two. Keeping in mind that no ECU from Japan is going to be tuned for 98RON and for all possible modifications, probably not going to make a huge difference but...?
Silver Auto Service are currently developing a remap system that will be done with live runs on the dyno, however the tool they need to do it is currently in Korea and they cannot confirm when the tool will arrive. Long story but as soon as there is an update someone will post something on the forum.
Aside from Mine's, Dolphin, Impul & Nismo there are also aftermarket ecu's available from G-Zone, DMY Racing.