Hmm... Caution is in order. SGRAM is expensive stuff and I haven't seen it on a card in a long time.
Though, to answer your question, I believe SGRAM does offer better performance.
Case in point:
My first Accelerator card was an I740. Nothing to be proud about, I know.
But my bro was using a Permedia 2 card. The I740 had 8MB SDRAM and the Permedia had 4MB SGRAM.
Now, the I740, as bad as it is, is a more advanced chip than the Permedia 2. It even includes implemented colored lighting. granted I could do higher resolutions, but that Permedia 2 outperformed my card in many situations.
Just my two sense.
The Jolly Wizened Oaf!