The upgrades that you should go for are the sidestrips and the balls. Tornado changes balls all the time, so if you want to keep competitive, you need to get the latest ball. The newer sidestrips are a lot lower profile, so these are a must (helps with passing practice as well as banks)
More costly are the newer men... Tornado redesigned the foot on the newer man, changing the angles, grip, as well as the lipped ridges on the very side edge of the foot. This effects brush passing (and anything else off of a backpin), as well as chips, banks, and rocking the ball when in a front pin. Maybe you're lucky and already have the new men on the table?
The playfield surface, handles, rods, & bearings are also different, but not worth the cost of upgrading IMO.