Smith, 17, finished tenth in the championship last year, becoming the first Briton for 18 years to stand on a 125cc podium at the French Grand Prix.
He has switched over the winter from Repsol Honda to replace 250-bound Mattia Pasini at the Polaris World Aprilia team and was quickest on the second and third days of the test at the Spanish Grand Prix venue.
But it was his 1:46.869 on the final day - the best time of the entire pre-season at the circuit so far - that got heads turning. He even had time for a light crash, which failed to disrupt his excellent progress.
Season 2008
125s: Spanish duo join Derbi
14/12/2007 AT 11:52
Leading the chasers was world champion Gabor Talmacsi, who was trying out Aspar Aprilia's 2008-spec bike for the first time. His best time over the three days was almost three tenths of a second slower than Smith's though.
Simone Corsi was third quickest on his Fontana Aprilia with new Ajo Derbi signing Mike di Meglio and Aspar sophomore Sergio Gadea the only other riders to break the 1:47 barrier.
Sandro Cortese, on his Caffe Latte Aprilia was sixth quickest with Pol Espargaro ending his first test on a works Derbi in seventh.
Stefan Bradl's Kiefer Aprilia and Dominique Aegerter's Ajo Derbi completed the top ten.
Britain's Scott Redding, 15, who was trying out his full GP-spec Blusens Aprilia, was 12th fastest. That put him one place behind Esteve Rabat, who was the best KTM rider on his Repsol version.
Rabat though, had an unsavoury end to his test as he crashed heavily with just an hour of the final day's running to go - fracturing his collarbone and being forced to potentially miss the first two races of the season.
Season 2008
Hayden demands Honda improvements
01/03/2008 AT 20:11
Season 2008
Stoner slams Losail gravel traps
01/03/2008 AT 14:36