I recently added Dons rod w/ 22lb. spring and the Pachmeyer grip to my tac .40. Wow. Between the two, it was like a whole new gun.
In terms of recoil, I do think the stiffer spring took some of the "snap" out of the .40. On the other hand, comparing to my XD9 service, I know that the added weight of the tactical did more to compensate for recoil then the stiffer spring. Basically, between my service 9 w/20lb spring and my tac .40 w/22lb spring, I notice very very little difference. I also think the tac points a bit better, and is more accurate to boot.
Now, with the pachmeyer grip, I was also impressed. For the longest time, I thought an aftermarket grip on the XD was silly because the grip is so sweet to begin with. Got the pachmeyer and accidently put it on upside down..but it felt great! Put it on right side up (pachmeyer lettering right side up) and didnt like it nearly as much. Upside down pachmeyer works for me! The grip alone on the .40 makes my 9 and 40 more difficult to compare to each other...I am seriously concidering another pachmeyer for the xd9.
Long story short.. Dons rod looks better than the stock rod (for what thats worth) and the stiffer spring does help, but it doesnt reinvent the wheel. I had about 4k rounds on the stock spring so it was due to be replaced anyhow (maybe not). For a brand new gun, I wouldnt make it a priority.