Concorde:
"While commercial jets take seven hours to fly from New York to Paris, the average supersonic flight time on the transatlantic routes was just under 3.5 hours. In transatlantic flight, Concorde travelled more than twice as fast as other aircraft - other aircraft frequently appeared to be flying backwards."
Concorde range: 4,500 miles
Maximum fuel load: 210,940 lb
Maximum taxiing weight: 412,000 lb

About 1/2 it's weight in fuel.

So with suborbital rocket you could be looking at about 60% to 70% of it's weight being rocket fuel and travel the same distance in less the time.
Obviously launching from a mothership from similar to SpaceShipTwo [instead a rocket from the ground] would make more towards 60% side.

But in theory one should be able lower the percentage of fuel needed by lower than 60%. One could have a mothership which flies faster and higher. Or one use something like a Maglev. But:
"The highest recorded speed of a maglev train is 581 km/h (361 mph), achieved in Japan in 2003" http://en.wikipedia.org/wiki/Magnetic_levitation_train

And even 400 mph simply isn't fast enough, instead you need a MagLev that could get over 1000 mph to get something better than what a mothership can provide.
But one could build a maglev track that was designed to operate in the 300-400 mph range- thereby not need a quantum leap in MagLev technology- which could launch a mothership. Of course, in this case one is mainly saving fuel use by the mothership which of course is more efficient than rocket engines, and I would guess this fuel saving wouldn't amount to much- perhaps 1-5%.

Such a maglev would be best at higher elevation- say over 5000'.

Btw, highest airport in America is 10,010’ elevation:
Possum Creek — Glenwood Springs, Colorado
http://www.mountainflying.com/colin_5.htm

One of the longest and highest in the world is:
"Bangda, Qamdo, Tibet, is one of the highest airports in the world, with an altitude of 4,334 metres (14,219 ft). At 5,500 m (18,045 ft), runway 14/32 is the longest public use runway in the world. The low air pressure at this altitude makes a higher takeoff and landing speed necessary, and therefore a longer runway."

A maglev at any elevation would allow one to take off with higher gross weight, but at higher elevation you need a higher take off speed than you would at lower elevation.
So, say at 10,000' a mothership fully loaded might need as much as 250-300 mph to get into the air.

In other words if you have mothership which has gross take off weight at say 2000', it couldn't take off at that weight at 10,000' [not even at 3000'] but with Maglev which launches the mothership at say 300 mph, it could have as much [or even more gross take off weight] at 10,000'.