Have 4k into my ls6 and run 11.53 naturally aspirated going easy on the start... 10k on an L's can get you 700+, And run low 10s. So I am questioning the your faster part too but it can happen. Plus L's engine take nitrous amazing and are reliable on a 125 dry/150-175 wet shot. Harris makes a good set up for $400.
As for the weight change I would need some help from you since my knowledge is in the Ls engine world, but the L's are aluminium block. If fact when doing my v6 (iron block) to v8 (ls6) swap I lost weight.
L's engines are amazing and get even better when you get a lsx block (427). Around here there are a lot of Saturn Sky's and solstices running with L's engine, they basically have to build the car around it... stupidly fast.