I think I'm probably the most qualified person to address this question, since i was one of the first,if not the first person to perform this modification. I'm also the person who wrote the instruction steps steps for making the modification.
Specifically, I removed 2.75" or 2 3/4" of unused upper Weaver Rail from the TAC15i crossbow.
This was done for two reasons. The first is that I found through a great deal of shooting and observing that when the crossbows cable comes up out of the slot it sits in at full draw, always one of my upper vanes would make contact with the under side of the upper weaver rail. It was impossible to measure how much interference this was causing in arrow launch, but I felt it couldn't be a good thing.
The second problem was that it prevented me from being able to shoot standard three fletch arrows which meant I was unable to test other higher quality arrows unless I re-fletched them to the PSE TAC15
format of 60 x 120 degrees. This was never a superior vane configuration to a standard three fletch, it was an engineering necessity created by PSE's engineers to be able to launch an arrow fro their crossbows design.
I've since had the opportunity to work directly with dozens of TAC owners (both TAC15
and TAC15i owners) that have gotten rid of the Whisker Biscuit Rest, swapped to the QAD Rest and modified their upper Weaver Rail as outlined. Without a single exception each person who has performed the changes has experienced a much cleaner, problem free arrow launch with much better down range grouping results. Many are also now using the new Aerobolt II arrows from Firenock with great results.
I personally use both PSE TAC arrows and Aerobolt II arrows. I can shoot any vane configurations from my crossbow with great results. This does not mean I can switch back and forth without re-tuning my windage or elevation for whichever arrows I'm using. The Dynamic Spine Characteristics of the Aerobolt II's are different than the PSE TAC arrows, so they will fly somewhat differently, but once the Whisker Biscuit is no longer interfering with the flexing of the arrow shaft and no longer causing uneven friction on the shaft and vanes, you will no longer experience occasional flyers or uneven grouping results.
Getting rid of the back end launch interference cleans up the launch interference and provides a silk smooth launch cycle that allows the spine of the arrow to perform the way it was designed to perform.
My scope is a 10.5" Leapers 3 x 12 x 44 compact, so my bases and rings sit an inch behind where the cut is made in the upper rail, so it presents no problem what so ever.
My last comment is the fact that since the upper rail is a separate piece from the upper unit, I could always purchase the piece of the crossbow that it's attached to and replace it, if it had not worked out to my satisfaction. That said, I have no plans to be replacing it.
In the event you wish to do the conversion in steps as opposed to all at once, the biggest gain in performance begins as soon as you replace the Whisker Biscuit. The rail can be done any time or not at all. TAC owners that are looking for the most accuracy and to eliminate some of the problems that I've already described will do both changes.
Please let me know if you're going to perform any of these changes because I can provide both pictures and some additional advice that will make it easier and avoid any mistakes.