Ugh, no.
voter with 1000 HP throws votes worth 30k HP for witnesses
No, he throws (up to) 30 votes of 1000 HP for different witnesses. His influence with each cast vote is still 1000 HP.
while a voter with 100 HP, rather than having 900 VP less than the VP the 1000 HP voter has, only casts votes worth 3k HP, which is a 27k VP difference
The sum of absolute values of votes is a meaningless number. The smaller voter has 10 times less HP than the bigger voter and therefore has 10 times smaller influence in the election. He still needs to only increase his stake by 900 HP to reach the bigger guy, not 27k HP.
First of all, top 20 are selected based on being within first 20 positions when ordered by power of their backing. The absolute vote values have zero meaning. Even if the last one of those 20 had a 0.000001 VESTS more backing than the 21st guy, he'd still win. Backup witnesses are running at the speed proportional to their backing (so bigger absolute vote power means more speed), but they win the race at the frequency proportional to the relative weight of their backing (so a guy with 1000 HP backing will win 10 times more frequently than 100 HP backed guy and the guy with 30k HP backing will win 10 times more frequently than the one with 3k backing).
I could go on how the system you're proposing would make it possible for someone with relatively low stake to block any fixes and changes that require hardfork, or how the only logical way (at least for whales and orcas that have most to lose on bad governance) would be to split the vote for 15 witnesses (17 around hardfork), how there would be need to constantly keep an eye on any changes in the votes to shift vote to next acceptable witness in case one of those you've chosen fell from top spot, how the notion of "wasted vote" and "voting for lesser evil" known from normal elections would be introduced to the system. But that's not necessary. The 30x multiplication of influence does not exist, therefore the conclusions derived from it are unfounded.
From the election standpoint it would be best if votes were not limited at all, however that would slow down updates of voting power and would pose potential attack vector. 30 votes represent good middle ground. They let you chose two sets of 15 witnesses (each poses consensus majority if elected) - the ones you actually want to represent you, and the others that are still somewhat acceptable but are more likely to be selected to the top positions. This way you vote for your representation and your choice is wide enough to be stable - not require constant updates due to votes of other people.