I think you have some of the theory correct, primarily setting 6th gear to optimize the peak power for straight line speeds. However, I get that you are then simply setting each gear 1st - 5th to be "close ratio".
I'd like to offer a different view;
First, there are two numbers; Torque and Horsepower. Horsepower is ficticious, but Torque is real. Torque (lower RPM "umph") is what truly accelerates your car. Horsepower (higher RPM revs) helps maintain those high speeds. If you are not using BOTH numbers in your power-band, you are losing time.
Your FINAL driver gear is extremely important. All other gears are based off this gear. Use what it offers. The higher any gear ratio, the more torque the engine will deliver to the wheels, and the more torque through the wheels equates to greater acceleration. A gear can simply be thought of as a "torque multiplier". A gear of 1.0 on an engine with 100lb Torque, will deliver 100lb Torque. A gear of 3.55 on that same engine will deliver 355lb torque. (not factual, but accurate enough)
6th gear is an easy choice as you say, but 1st gear is used for standing starts and/or leaving your pit stall, if you have either in your race strategy. If not, I agree with setting your 1st gear taller to accomodate rolling start speeds and/or the slowest corner on the track.
Why use 1st gear instead of 2nd? For the very reason you are saying NOT to... wheel spin. That same wheel spin means the rear tires are delivering massive amounts of torque. Torque = acceleration if you can modulate it properly. So... modulate it... and get the benifits from the higher gear ratios of 1st gear and that final-drive gear. Punch out of that slowest corner (ever notice how that corner usually leads onto the longest straight?) and pass your competition. Set that first gear so you are not punching through it too quickly, but also so that you are using MAXIMUM TORQUE when you need it most.
2nd - 5th are NOT as simple as nice even gaps either. In fact, if you have even gapped gears, you are losing time.... at least you will be too me
2nd gear should be tailored to the corners of that speed... 3rd to those other corners... 4th to those corners, etc. Utilize that peak powerband so JUST as you are completing the corner-exit phase, you are just entering that peak power. Gears will no longer be nicely spaced, but you will shave a tenth off each corner on the track. 10 corners later and you are a second faster simply by optimizing each of your gears to their specific corners.
Now, what about those uneccesary shifts? Each time you shift up to the next gear you are not accelerating forward. If you are making a shift into 5th gear at the end of a short-straight only to mash the brakes a second later, then you have wasted the time to upshift and not even used the next gear. Increase 4th gear and avoid the shift all together, right?
Make these changes and I bet you that your 6th gear will change too. You will carry more speed down that same straight with no extra effort on your part.
For more info, see http://www.race2play.com/forum/list_posts/4130