SIP and PF are pretty close to one another in terms of cut and gloss, but even though the the .3 doesn't seem like much on paper, it sometimes is all you need to get rid of a couple swirls completely. This is why it is nice to have multiple products in your arsenal.
This was the case on this particular car. It was black and showed everything. I tried the PF with orange and even though it removed the majority, it didn't remove all of the swirls. There wasn't much left of the swirl after the PF, but you can't get by with leaving even the faintest of a swirl behind. Once I switched to the SIP, it cleaned them up nicely.
Another thing to note is just because a polish or compound is shown on paper as being close to each other, it doesn't always mean they are. Different factors come into play other than the size of the abrasive such as how long the abrasive needs to be worked before it breaks down. If an abrasive breaks down slower, during the time it is breaking down it is removing more material than if it were to break down fast because the abrasive is larger for a longer period of time thus removing more material than when it is smaller. Not sure if this makes any sense or not, hope it does.