I have a first generation Walther P99 AS in 9mm. I bought it based on feel alone in comparison to a number of other available pistols. In other words I never shot it before I bought it. I have since shot it and I shoot it reasonably well. Lately I’ve been thinking of getting a second pistol to have as a back up. It makes sense to me to buy the same gun as a back up. Here’s where my question comes in. Should I actually shoot a variety of guns to see which I shoot best with? As opposed to sticking with the P99 based on feel alone i.e. Are some guns inherently more accurate or is it mostly a training issue. I have narrowed the field to a few guns that I like based on how they fit my hand. I’m thinking of trying to find a range that rents most of these so I can try them and see if one shoots better for me. If one stands out I would buy two of them and trade the Walther ( unless the Walther is the better shooter). Here is my short list:

Walther P99
Sig 228 / 229
Sig 226
HK USP / HK USP Compact
Springfield XD9 / XD9C
FN FNP9

In summary my question is should I bother trying all of these pistols or can I get maximum performance from a set of Walthers through training alone?

I always suggest trying to rent and shoot as many guns as possible prior to purchasing.  If you rent some of those, you may find some that you absolutely love, and others that just don't do it for you.  You may find one that you like even more than your Walther.  Think of what the most important features and characteristics are to you, and then make a list.  Some of those are striker fired, some are DA/SA, some have decockers, and some have decockers and safeties.  Some are single-stack (Sigs) and some are double.  Are you planning on concealing it?  The only two on your list I have experience with are the USPc 9mm (my daily carry gun) and the XD9sc.  They're both excellent weapons, and I've really only heard excellent comments on the others you have listed as well.  So yeah, rent as many as you can and, in the end, get what makes you feel the best.  If that's a second Walther, awesome!!  The only thing better than one gun is two guns...or 3, 4, 5...  ETA:  Forgot to say, keep in mind that range guns will have several hundred or thousands of rounds through them, so they will be very well broken in, and the trigger will have probably smoothed out a lot.  

As one who has shot all of the ones you listed & also owned several of those, please allow me to add my .o2:

The H&K will be THE most accurate gun you can buy, generally speaking. Having said this, this doesn't mean you (or someone else) will shoot the H&K more accurately than the Sig or Walther or XD or whichever.

I would suggest you load up some factory ammo & head to the range. Good odds that you will find all of those guns at the firing line sooner or later & with factory ammo, most shooters will quickly offer you a mag or two to run through their guns. This is simply the best way to rate your future purchase, IMO.

My .o2  

By all means, rent and try out a variety of pistols. Consider the S&W M&P as well. I was looking at several of the same pistols you are, and was convinced to try an M&P and it was exactly what I was looking for