Tooofst:
A thought - a hp increase with +4 on the VEC1 means you are running to lean, not enough fuel. What is your air/fuel ratio. Try to get further enrichment through jetting or modification of the air temp/coolant temp sensing circuit.
You can make all the hp in the world and if it is at the wrong air/fuel ratio you are putting your engine in jeopardy - detonation at wot is not always apparent. As your car runs lean you will loose hp, on the other hand a car that is running a little rich will not loose much hp and it is much safer on the bottle. I would be very careful of going into the minus range on the VEC1 when on bottle.
I've been playing with nitrous and have over 30 pulls trying to dial it in and I'm also using the VEC1. Just a couple of thoughts.
1. Always monitor air/fuel. This should be as important as the hp/torque. Make a base-line of the air/fuel off of bottle and on subsequent pulls always try to keep the air/fuel close to the stock setting in the 3000-5000 rpm range.
You will want to shift at 4400-4600 on bottle, as the torque should peak between 3500-3800. Your best setting may not be the max hp/torque. You want the best hp/torque at a safe air/fuel ratio. There is a book available from speed shops called NOS. It is good reading
When you plot the air/fuel curves they tend to lose some detail because their x-axis range is so small. See if the dyno owner will give you the program so that you can play with the curves when you have some time. It is very interesting to plot just the air/fuel curves at full page. At that resolution any changes you make are very apparent.
2. Pull all plugs after any changes. Air/fuel is an average and any one cylinder can be lean even though the air/fuel shows as acceptable. The air/fuel sensors on the dyno can go out of calibration as can the enrichment circuit of the nitrous - always compare the air/fuel of different sessions with some common baseline.
3. Many people are successfully running the stock Chamkpion plugs, however, this plug has a design that is terrible for nitrous. It can and will act as a glow plug and actually cause detonation. The use of a NGK 7252 will not result in any less hp off of bottle and will safeguard against the glow plug effect.