Although, the nastiest istlanum I've witnessed was just as crazy as a grammodes... I have seen some peaceful istlanum and not a single peaceful grammodes! Even the least aggressive female grammodes I had was causing problems in a community tank against convicts.
The bigger the grammodes get, the scarier. Istlanum tend to get a bit chunky once they reach a certain size, kind of like Midas cichlids.
Grammodes are well-made, thin agile body, large powerful mouth, and a very intelligent fish. Like you mentioned, they have a bizarre fighting technique that subdues much larger fish! Many say the more shy the cichlid is, they often are much nastier in personality.
A neat little video I found a while back, this dominant male grammodes (in a overcrowded tank) is holding off a istlanum pair!
Link:
https://www.youtube.com/watch?v=YkvvBS5-7Uk
And an interesting quarrel between a Grammodes and a Bartoni. This individual grammodes is surely different from what you said.
[video=youtube;YNKufcuYfPQ]https://www.youtube.com/watch?v=YNKufcuYfPQ[/video]