Question regarding Mizuno demo iron (6i) vs. purchased mizuno iron set - different swing weight?

Hopefully this is not a strange question, but i've been looking to go from my mp-62 to full blade mp-68's and using the mizuno demo club (6i) i played around with the DG, PX, and KBS shafts.  I found the KBS shafts the "smoothest" feeling for my swing out of the bunch.  My mp-62's that I have had for almost 2 years had DG s300 shafts, and I never really got a good feel for the shaft, and sometimes i would get a higher than "normal" ball flight.  I know that the KBS tour shafts are supposed to launch higher with more spin, as opposed to the DG more "penetrating" ball flight, but I found that the ball flight with the KBS shafts were much better for me overall.  I tried both the stiff and x flex kbs shafts with the mp-68 demo head and actually found that i loved the x flex shaft, i could really go after the ball more effectively.  After several days of trying all the different shafts, i was told to use the mizuno shaft optimizer, which i did, and it recommended as one of the chocies, kbs tour X flex soft stepped once.  However, since there was no soft stepped kbs tour x shaft in the mizuno demo kit for me to "try" out, I was extremely hesistant to purchase.  I went ahead and ordered the x flex kbs instead with no soft stepping since this was the shaft that felt the best for me when trying all the different shafts.  I received the clubs, and I love them, however, the question that I have is, for some reason, the clubs feel a bit "heavier" than the demo mp-68 club with the same kbs x flex shaft in it?  Is this a swing weight issue?  My clubs are all standard lenght, lie, loft, etc., but could the demo mp 68 6i with the x kbs tour shaft have a different swing weight than the purchased mp 68 6i x kbs tour shaft that accounts for the "heavier" feeling even though the weight should be identical?  If so, what impact can I expect this difference to have aside from just "feeling" different.  Also, any way to get the same "feeling" I had with the actual 6i demo club?  Thanks.

I don't know if this will help you or not, but I recently (well, last September) re-shafted my MP-57s.  I bought a demo iron from a bin at Roger Dunn cheap, and re-shafted that to the shaft I was considering (KBS Tour Stiff, soft stepped once).  I liked it, and re-shafted the rest of my set... except the 6-iron from my set, which is in my closet with the original R300 shaft.  If I decide to re-shaft again, I'll re-shaft that to find the one I want, and then I'll re-shaft the rest of the set around it.  But I haven't noticed any different feel in my "new" 6-iron than the rest, even though that came out of a demo iron (but not a demo kit).

the demo club I am referring to is from their fitting cart, where they can unscrew and screw in different shafts to the club head easily.  I'm wondering if the fitting cart 6i is a tad bit different (not a fixed shaft, but removable shafts) accounting for the different feel from the actual production 6i?

Not sure, but are you using the same grips as the demo? In the Mizuno fitting carts, the shafts have a color on the butt end - did the demo shaft have a white or yellow butt? A yellow butt is +1/2", white is standard.

If all is the same, I wonder if the interchangeable heads really effect the swing weight significantly... I'll check and see tomorrow when I'm at Golfsmith.

Yes same grip, i specifically wanted the exact same setup because it felt so great hitting with that demo club, so the grip is the standard golf pride m-21 grip made specifically for mizuno.  Also, the shafts that I tried were the standard (white at the end of grip) shafts. I am 100% sure because I helped the guy find the shaft that I wanted to try out.

maybe it's my head playing tricks on me or i just happened to have played too many consecutive days when i received them and put them into play?  I haven't picked up my clubs since monday to rest, and i guess will give it another go tomorrow or Friday to see.