To be quite frank, I found that Opera Turbo rarely sped me up noticeably, and sometimes slowed me down. Just one user's experience in North America today, but there's definitely room for improvement.
If your net speed is more than 512 KBPs you are not going to notice any improvements whatsover, rather (as you said) you might notice delays due to the increased overhead. However, on slower connnections the effect is marked.
This is because a 100 KBps connection can download 100KB in 1 sec and 200 KB in 2 sec. On the other hand a 10 KBps connection will require 10 sec and 20 sec respectively. And, in this case, the effect of Opera's compression becomes noticeable.
P.S. What Opera is doing is similar to what www.onspeed.com does. I used to use that during my dialup days, and it was a big big help.
