Isn't this kind of a known issue in Linux land? As much as it has improved, latency has always been the bane of audio applications in the Linux kernel. I remember in the days of kernel 2.2 that even XMMS would stop playing any music if I started using more than one or two applications. Recently I got one of those cheap USB interfaces to connect my guitar. I spent some good 4 hours changing the kernel to "low latency"…
Trying to make Pulse work is a big mistake. The first step on any Linux setup is ensuring that it isn't installed. One can get very short latency out of Alsa, up to the point where the hardware becomes your bottleneck. But that's extremely processor intensive, and won't work well if you try to share the dsp with several processes (if you want to get that extreme, I'd recommend you get extra hardware for exclusive use…
About the interface. I doubt that was the problem. When I got jack to work, I was getting 2-3ms latency between input and processed output. I did get the guitar effect application to work, I just thought it was too inconvenient to be forced to be aware of "what-application-uses-what-sound-system-and-when-I-need-to-flip-the-switches".
In any case, my goal was to have a alternative that could be (a) cheap and (b) convenient if I wanted to play with my guitar and have some DAW tools, not to see how low I could bring down latency in a linux system. The lesson learned is that it can be cheap, but not convenient.
Ps: did you go to Unicamp?