Im not sure about the Ristretto, but I have the Latte (a couple of versions up). *Ill assume the stock filter is a pressurised basket like mine.
I never had success getting more than a 12 sec pull using the pressurised basket, so I took the advice found on some of the threads here and bought a non-pressurised (Krups) basket. *Im still fiddling with grinds and tamp pressure, but am now getting ~30 sec pulls and excellent-er coffee.
I would expect the Ristretto has similar, if not the same, size basket as the Latte... but cant say for sure.
Also, before the Sunbeam I had a Gaggia Carezza (youll notice I cant spend a lot on machines at the moment :P), which has a small boiler as well... with pretty much the same results as you. *Thats one of the reasons I gave up on it.
Hope that helps a little...