I used the TG Derby archive to look at how a horse's path in the first turn correlated with how he ran in the Derby relative to his 3yo top. The sheets in the archive for the Fusaichi Pegasus Derby are minus path info, but for the remaining horses in the sample, here is the data on how the median horse in each path ran relative to his top:

1W - Off 4.6 (4.2)
2W - Off 3
3W - Off 1 (1.5)
4+W - Off 2.7

Sample size is obviously an issue (around 30 in most cases, but only 19 for 4+W). In a couple of cases, the distribution was lumpy right around the median. The numbers in parentheses for 1W and 3W are medians adjusted to fit better with the surrounding data points.

In any case, if you assume that one TG point at 10 furlongs (or marks) is about two lengths, and that being one path wider results in ground loss of a length, it's better to be a path further out to gain one half a TG point. Based on that, it looks like 3W is the place to be in the first turn.

I assume that the advantage of being 3W probably relates to the likelihood that you were not troubled in the run to the first turn and are able to find clear sailing down the backstretch, but I'm just guessing. This data would be consistent with the number of winners that have come from outside posts in recent years.