Final 0.9 version is out, and the new and improved staging options offer room for quite a bit of improvement on previous efforts
Here's the results of a straight vertical launch of an embiggened and yet somewhat simplified - except from the initial boost stage at any rate - version of the previous Il Tempo Gigante (craft file attached, by the way):
That's right, over 9500m/s on a vertical launch. I'm guessing this thing could probably break 10k m/s on an oblique launch*, though this thing is heavy as hell to steer so I'd hate to actually try it. It's made even worse by the fact that this packs exactly zero SAS modules once the boost stage is gone - It is stable enough (well... barely) in a vertical launch, presumable due to the length of it, but if you get even a bit outside the desired flight path it just doesn't have the steering torque necessary to recover. As before, this is stock parts only.
Oh yeah, and a lunch pad shot to show the scale of the thing:
The stuff at the top is actually part of the first stage, to help get it clear of the tower, and are what I refer to as the boost stage. Couldn't put them on one of the 'legs' because that causes bad spinning that eventually tears the rocket apart, but right at the top they're actually far enough above the lower parts that they can fire without damaging anything. The whole assembly there is gone before the rocket hits 3000m altitude.
* The old version went from 7500m/s to 8200m/s when going for an oblique flight path, but gain would likely be less from this as the higher speeds take gravity mostly out of the equation for a greater part of the run. Around 500m/s seems a reasonable expectation though.
[attachment deleted by ninja]