Closed loop means the computer is reading the oxygen sensor and adjusting the air/fuel ratio to keep it at a certain specific setting.
Open loop means the computer is not checking the oxygen sensor and is running a pre-programmed setting for that temperature, mass air flow and so forth, almost always rich, certainly so if you are under boost.
Your car doesn't appear to use an exotic oxygen sensor so most likely if your computer is still closed loop under moderate boost then you are running efficiently.
Someone here can tell you how to adjust you Scangauge to check if you are open or closed loop, I don't have one so I don't know.
I'm developing a spreadsheet calculation using my car (1994 Subaru Legacy) and estimated BSFC curves (from steady state mpg estimates) to determine the most efficient method. I need to play with my spreadsheet more, but I think it the ideal method will depend on the upslope/downslope steepness, and the BSFC vs hp curve of the vehicle in question. I'll post up what I find when I'm happy with the numbers I have.
The ideal downslope is one on which you can coast and maintain your speed.
More and you want to slow down at the crest and let your speed increase downhill use top gear fuel shutoff if you need to slow down more (foot off the gas).
Less and you want to pulse uphill to a slightly higher the top speed so you can coast further downhill.
Your objective is to always pulse uphill if possible and keep your speed as close as possible to you average.
When they get too steep to stay at speed in high gear (downhill) with fuel shutoff you may have to use a lower gear to keep your speed down. It may also be better to climb in a lower gear if they are steep enough to keep you from manitaining speed uphill in highest gear. Always use highest gear when practical, and go as fast downhill as you feel is safe.
When they get too steep to stay at speed in high gear (downhill) with fuel shutoff you may have to use a lower gear to keep your speed down.
No. Do not do that. At that point, you have absolutely nothing to gain by using additional engine braking unless you need it to avoid brake fade (something I've never experienced even while towing a 6000 pound camper on mountains, but I digress). You do have something to lose -- extra wear on the engine and clutch and synchro, or if you rev-match, you'll spend gas and still get extra engine wear.
If you're already in DFCO and need to slow more, use the brakes.