Making my songs louder after mixdown

What have you done prior to this? What kind of compressing have you done? That 0,1 dB surely means peaks, how is the average RMS level, which is the most important when talking about loudness? It may well be you should have much, much more headroom left when starting to make tracks louder, depending on what you have done this far.

Does the Waves L2 Limiter work on Sonar?

Yes

You should be leaving at least 6dB (peak) after mixing in order to give your mastering tools some headroom.

Kalle is asking the right questions above - what have you already done and what are your RMS levels?

Mastering is the next step once you've mixed to your satisfaction. You can use Sonar or better yet, a program like Wave Lab or Audacity to master. The loudness game can sometimes be the wrong way to go with certain music. Mastering , which is a whole other skill, is sometimes best sent off to someone with the correct tools. You can quickly trash your songs with the wrong compression/ limiter set up. Wave lab has a Loudness Maximizer that works for small boosts of say + 4 Db. I have also had good results using Sonars multi band 64.( comes with 8.3. ) You do need to know how to apply it. Mastering takes as long to learn as each other step in the recording process.