I guess you can land kills in a PT boat. Funny thing is, no matter what I've tried, I've never done it. One time I had five kills. I knew that just getting more than 6k from the cons wouldn't give me a landing so I tried getting out of radar range of the enemy port.
I headed out to sea and kept an eye on the map with the radar range icons displayed. After maybe 20 minutes or so, I thought I was out of radar range. I ended sortie and was told I had ditched.
Come to think of it, in the over three years of playing, I don't believe I've ever seen an announcement of someone landing two kills or more in a PT boat.