I HAVE to chime in, I have used SuperCSDIv4 ROMs for over a year, every day, and recently switched to another awesome ROM
I have only had a G1, no other phone. I have had one for 3+ years. All that time I have only tried HTCclay's Superfast. a few variations of SuperCSDIv4 (90% of my use) and now Superbler with RAM hack.
I have yet to see a downside moving from the Donut based SuperCSDIv4 to Froyo based Superbler. Not even speed is an issue. I really recommend you check this ROM out! I have to disagree with the poster who mentioned that people were forgetting how fast Donut ROMs are (and I have been using -arguably- the fastest one) compared to Froyo ROMs. All in all I think Superbler with the RAM hack is just as fast, some things take longer and some are faster. I have the same apps on both phones and some things definitely open and run more quickly on the Froyo ROM.