Yeah I think its reasonable to say that 100lbs is significant. The 200lb difference between the Ecoboost/V8 mustang is very notable in the front end of those cars, and its a smaller % of the whole weight.
That said, K series motors are perhaps just as or nearly as well understood as LS motors. I'm not a fan of honda cars by almost any measure. However, it is different, there are options it is an extremely abundant motor, the T56 that normally comes paired with an LS is getting harder to find in good condition, and if we are honest...isn't a nice transmission to use.
By way of comparison 500HP in a K series, is completely achievable. May not be recommended but there are people out there on stock internals making 500hp. Can you make 500hp easier on a LS? probably, is the whole package cheaper? Maybe. But, you almost certainly can get a K-series for cheap. 200 bucks at a junkyard, throw 1200 of internals at it and you still have money for lots of bits and bobs.