• OBD-I Reversion:
The main reason why people revert to OBD-I is to facilitate the switch to a standalone, user-programmable aftermarket engine management system.
There are very few (if any) performance gains that come from reverting to OBD-I. It only facilitates the installation of systems that provide more tuning flexibilty.

Furthermore, the standalone engine management is designed for use on very high-output, "radically" tuned motors. If all you intend to run is bolt-ons and a built head, a piggyback controller like Apexi's VAFC is much more cost effective.

• On the topic of "Chipping": The outstanding majority of performance tuned ECUs simply tweak the ignition timing curve... advancing the ignition timing. The gains are fairly mild unless you get into custom tuning to match specific modifications.

And if you're considering the use of nitrous oxide, the advanced ignition timing from a "chipped" ECU will increase the risk of detonation while using the nitrous.
Every manufacturer of nitrous oxide injection systems warns against the use of "chipped" ECUs in conjuntion with nitrous.

Basically, ECU mods are a facet of performance tuning that supports the modifications to the engine. If you're planning on bolt-on modifications (intake, header, exhaust, cams) the stock ECU will be able to handle it.

Hopefully, this clears things up a bit.