That's a very difficult question to answer in the manner it's been asked. Before I could even attempt to answer your question you'd need to be much more specific in clarifying it.
At what distance are we talking about?
What type of groupings are you experiencing with your four vane arrows?
What type of a performance improvement are you looking for or compared to what?
I'll state the improvements and benefits that typically were supposed to come from this modification.
Prior to the modification of the upper rail, your crossbow would have likely caused one or more of your arrow vanes to come in contact with the underside of your upper weaver rail once every some many shots. This would have caused a flier condition where you would periodically have noticed one arrow a few inches outside the spot you were shooting at. You would have no explanation for this event because you would have felt certain your crosshairs were on center when you pulled the trigger. This flier condition is a direct result of your arrows vanes not having good clearance and the launch causes the back end of the arrow to make contact with your Weaver Rail. This results in a disrupted launch process causing less than desired results on the face of your target.
Although you can shot 3 vane fletching configurations (and I do), in order to obtain better performance than with my 4 vane arrows, I need to re-tune my crossbow to shoot the three vane configuration arrows. They will not shoot or perform the same as a four vane, therefore my point of impact changes so my windage adjustment must be changed accordingly.
Three vane will fly at a slightly higher velocity because they have less "Drag" affecting the tail of the arrow and they weigh slightly less. This can account for as much as 5 or 6 fps. greater speed.
For the record, I have 2 dozen PSE TAC15
Arrows that are Spine Matched, Nock Indexed, and Weight Balanced that are perfectly tuned to my crossbow. I will not change these and these are what I shoot for hunting and a good deal of the time.
I also have 1 dozen PSE TAC15
Arrows that were build exactly the same way except they were done in three vane configurations. These also shoot perfectly once I re-tune my crossbow for them. At up to 100 yards, if any arrow is not within 1" or 2" of my aim point, then I know the problem was either a slight breeze or me, but never the crossbow or arrows.
Last, I also have a dozen of the "Firenock" Aerobolt II arrows, that were done in a 3 vane configuration. After receiving them, I Spine Tested each and indexed the nocks to the stiff side of the spine on each arrow. I then aligned my vanes with the stiff side of the spine and weight balanced each arrow to within 2/100ths. of a grain i weight of one another.
I can't tell you how well these arrows group, because I won't shoot more than one arrow at any given dot at any distance. These are my tack drivers and are reserved only for my most serious shooting. Once tuned to the crossbow these arrows are capable of same hole accuracy at almost any distance.
I haven't been able to shoot more than 1 arrow at my targets at any distance under 60 yards for over a year now, so grouping is something I only attempt at distances over 80 yards.
My suggestion, is that you work with a given vane configuration until you have it tuned to your crossbow. There are many factors which can alter flight performance, so there's a number of tuning changes that can be made to attain better flight performance. Temperature variations will always affect elevation and velocity especially at longer distances. Weight deviations will affect grouping performance at distances 60 yards or over. Unless your arrow nocks have been indexed to your arrows spine, you arrows oscillations are occurring in all different directions. This is a Dynamic Spine Deflection Timing Difference.
In simple terms it means your arrows are not flexing in the same directions and therefore it equates to arrow performance differences. It's the same difference as with a gun shooting precision matched ammo versus store bought off the shelf ammo. The only difference is with a crossbow and arrows the differences are greatly magnified.
Once you have settings that support the type of performance your looking for, unless you're into just experimenting, don't fix what isn't broke.
Hope this helps you understand why your question is not easy to answer as asked.