It depends entirely on the game. Sometimes scaling is very good, sometimes turning SLI off will bring better performance. If you're going to be doing SLI, it's only worth it if you are linking high-end cards, otherwise, the better solution is to buy a single faster card. I personnally think CrossfireX is way ahead of SLI ATM. It allows for up to four GPUs on the AMD 780FX chipset, and also introduces non-identical GPU combinations (for example, a HD 3870 and a HD 3850 can now work together) Scaling is also slightly better than SLI from what I've seen.
What cards are you planning on SLI'ing?
but the average seems to be about 50 FPS more when the same settings are used.
Err... You do realise that means absolutely nothing, don't you? What about if he's running Crysis at 20FPS, you're saying adding another GPU will bring a 50FPS increase. And what if he's getting 300FPS in Quake or something?