speeds on late 90's + up 29 fountain with single 500 efi and 575sc . also what kind of head room is in the cabin and what type of amenities in the cabin.

1996 w/HP500. Mine runs just about at 70 with a full tank of fuel, but it's the smaller step than later ones and the cockpit/engine compartment arrangement is different. You'll probably find the 97+ ones are faster.
As for the cabin - I'm 5'-3" and have to hunch over standing up. Sitting headroom isn't too bad. Amenities: V-berth with privacy curtains, settee just aft wrapping around to starboard (leather), porta-potti under front of settee, 'galley' to port with sink and cooler. There are a couple of mirrors, recessed fluorescent lighting and accent lighting.

Looking to port:

Looking to starboard:

26 Sonic,
Our 98' 29 single (500 HP) ran 75 with a 25 pitch LH hydromotive. Most of the 29's I have seen had the LH prop setup. Must've been faster.

As far as the cabin...plenty of lights. Mini-galley to port. Porta- potti in the center of the u-shaped settee. Storage areas under the v-berth. The water system was an option which we ordered but never used.