Created this post for documentation. The general understanding online is that the 98+ Malibu Response and Response LX has the diamond cut hull and the same great slalom wakes we know and love. 98 was the first year of the new hull, and 99 was the first year of the new upper deck/interior which features the rear trunk.
Here is the part that isn't documented as clearly (at least online, I'm sure the promos at the time were aware). To make room for the trunk in 99+, the engine and pylon were moved forward by 1.5" - 2". This drivetrain movement and change in weight distribution changes the wakes, acceleration, and top speed of the boat.
How would I know this? I have a 99 and my buddy has a 98, both response LXs. The slalom and trick wakes are only slightly different (hard pressed to say at all). But, his boat is over a full MPH faster than mine, particularly pulling a footer. I'll get 45.3 mph pulling front wakes, he will get 47 mph under identical condtions, and identical props (we even traded props to confirm). Furthermore, the foot tables are different, with favor going to the 98. It was a local 3 event footer that pointed this model year diffeence out to me after I said "I don't know why my boat is slower"
We put them on the dock side by side yesterday with the clamshells up and it was clear- you could see the power train placement change by 1.5"-2". So, if you are a footer who wants the diamond cut hull (hardcore, you really want a sportster though, or a DSII), get the 98. For a marginal slalom improvement, go 99+.