Uh...buy the best parts available.
Ok maybe not the best, I did skip on the $1,200 exhaust and went with a $500 one I think. But there's no sense in spending $400 only to find out that the $500 was better, and if it's not, it's only $100 lost. If it takes 30 credits to install, that's $120 wasted if you buy the wrong one (@$4/credit for working, approximately)

How I tune my A/F ratio:
I go into Photoshop, load up the dyno chart, copied off the website.

Then I zoom in as far as I can, and section off every 500 rpm interval (It only does 1k rpm intervals on the chart, but you have to tune every 500 rpm), find the intersection with the dyno line, make lines across the chart at each of the intersection points, approximate where the lines in-between would go, and then you have each line telling you how far over/under 13.5 your A/F ratio is.

Then make a line across at 13.5 on the chart.
(make a "X" with 2 lines in one of the boxes and then make a horizontal line, or just count pixels (there's 41 I believe between horizontal lines, so 20 above, 20 below the 13.5 line.)

I write down on a piece of paper, from 1k rpm to 9k rpm, +/- for each 500 rpm interval intersection. If it's 1 line above the 13.5 line, then that's a +1. If it's 4 lines below, it's a -4.

Then go to the dyno, and adjust each interval by the +/-. If it was at 68, and was a +7 (7 lines above 13.5) then I add 68+7 to get 75, which is the new setting at that interval. This is why I usually do a first baseline dyno at 80, so I'm dealing with one number to adjust, easier to not make mistakes.

Well... you asked