Under cabinet lights are sh!t, rip that plastic fronting off and get a plant specific bulb. If you are stuck for space and want to use t5's and t8's they also make them in one foot versions with seperate ballasts but they are mainly aimed at the small fishtank market. I wanted to make a veg chamber with them but always had the room for two foot or 24 inch long ones. Here is a link-
http://www.petplanet.co.uk/product.asp?dept_id=1255&pf_id=51571&co=fr
This runs a one foot t8 or t5, there is a little attatchment so you can run both. What you do is buy a grolux or fishtank daylight 8w flourescent, rig four of these and you are basically running a couple of two foot flourescents, penetration should be the same no matter what wattage as the bigger the wattage the longer the lights not the bigger the wattage the more powerfull the light, this is the point of t5's over t8's as the t5 is a more powerfull version of the t8. In my opinion t8's give better light coverage.
Seriously those undercounter light units are not aimed at plants, the bulbs are always wrong, go off the kelvin reading if the bulbs have one, should be somthing like 6500k or 2700k if its any good but you need the right one for the veg chamber and they are blue looking or white when switched on. If you got the money buy some seperate ballasted units with relecters and the right bulbs. I really think that the plant/marijuana industry sucks when it comes to flourescents and cfl's as the fish tank/plant market has been streets ahead for years now, even the aquatic plant fertilizers seem to be far superior than the plant ferts and they understand and diagnose plant deficiencies like it is an applied science.