Assuming everything's clean, lubed, well adjusted and fits okay, upgrading your bike's frame or Shimano components won't make much difference in speed. Upgrading your saddle might, if you can reduce soft tissue pain.

For component upgrades, I've found measurements help; I don't have to judge what might be small differences by feel. Fortunately I don't have to measure everything myself, either - others have measured for me!

Here's an old article about how to go faster on a racing bike:
http://www.sheldonbrown.com/rinard/aero/formulas.htm
There's no expiration date on physics so it's still helpful even though it's old.

The author points out that most of the resistance at racing speeds comes from aerodynamic drag. There's a table showing time savings from various upgrades. Here's where the article's age matters - you can't get most of those parts any more. But what may be pertinent to your situation is that there are no component class upgrades (Sora to 105) listed!

For faster tires, consider rolling resistance. For racing I use tires with low rolling resistance. I choose them by looking at this list, starting at the top:
http://www.biketechreview.com/tires_...sting_rev8.pdf
Of course, tempered with other info, like does my sponsor's shop sell them, flat resistance, price, etc.

In the end, basics for racing sucess are (1) Power (your legs!), (2) Racing smarts (your head!), (17) a basic functional bike in good condition. I've arbitrarily made the bike number 17 to make the point that there are other more important things, like good rest, recovery, motivation, teammates, etc.

Have a great time!