If you didn't know the difference, the feel would be slightly different as with any new equipment.

If you think/feel they are too light, any golf shop can sell you some lead tape and probably balance your clubs for you.

You might find you like the lighter feel.

Most can't tell the difference between D2-D4.  Interstingly there was a story about K.J. Choi going to get an off the rack set of irons at a Golf Galaxy becasue tour vans were'tn allowed at the Memorial last week.  He hit 4 or 5 sets, and picked one set because some of the irons felt uneven in all the other sets.  The set chosen was all D2, and the others varied from D2-D4 across the whole set.  Finished in the top 25.

Sign that man up to play the Princess and the Pea.

The 712AP2 irons with PXi 6.0 at 1/4" long with a standard grip will be built to a D3 swingweight if nothing is specified.

I heard that an amateur player cannot really feel the difference.

One guy told me that the difference is a dollar bill tied to the bottom.

Just that I paid a lot of money for the fitting and clubs.

I just want it right and hopefully use the clubs for 3~5 years.

I read that article on KJ, People were saying that miz is pretty spot on with swing weights.

I've read through a few articles that most amateurs can't tell a swingweight difference of less than 3 points.   D1-D5 is easy to distinguish, for instance and I have done some experiments with lead tape quite some time ago.   I can tell the difference in feel between a midsize Lamkin grip (67g) and a standard size grip (52g) - that is ~3 swingweight points.