From the Mike Davidson Announcement sIFR 2.0 :
I looked at the page and he replaced every word with sIFR ... even with full paragraphs and 300 word passages. Do not do this, please! sIFR is for headings, quotes, and other small pieces of text. In other words, this is a display type that emphasizes the rest of the page. A copy of the body should remain in the text of the browser. In addition, we recommend that you do not replace more than 10 blocks of text per page. Some more good results, but as soon as you hit 50 or so, you will notice that the processor and speed fall.
So this is not a good idea. In fact, sIFR is not needed on its own these days (Mike’s announcement since 2005). The new CSS3 font features are a much better way to bring awesome fonts to your users.