COWWs will have 2 -3 % antimony (maybe, more likely 2 - 2.5%) and very little tin. That is why they are brittle. Adding 2% tin will allow it to combine with the antimony forming the sub metal Sn/Sb which will then go into solution and stay in solution in the lead. The resulting alloy will be stronger than the individual metals and much more malleable. The resulting alloy will also HT very well, usually to a BHN of 18 - 22. That will make for a very good cast bullet that will shoot well at your 2400 fps. Terminal affect will also be much better due to the more malleable metal which will not shatter as much. With a good lube such as you are already using there will be no "leading" from the addition of 2% tin. Actually there will be less because of the better alloy produced.
Adding just lead to the COWWs will just dilute the amount of antimony present and whatever little tin there may be with out any improvement in the alloy.
You can certainly try it both ways as I have and have found what works best for accuracy and to prevent the bullet from breaking up at that velocity (2400 fps). I've found that COWWs + 2% tin then mixed 50/50 (sometimes 60/40) with lead is an excellent hunting alloy upwards of 2150 - 2200 fps. However, to push beyond that in 30s, 35s and 375s I've found The COWW + 2% tin alloy HT'd to perform best.