i would not lower the engien if you can just reach 4900 rpms.
trie a 19 inch viper ,i also use on my 175 etec and this props they fitt on this compo.
did you ever try to trimm the etec total out in calm see in the fine trimm,to reach more RPM , if yes did the prop got air?
in turns you have to trim down that the prop does not get air and to seed out off turns is allmost impossible with this hull.
you have to get your RPM up because this is not good for your etec.
send me an mail firstname.lastname@example.org
because it could be that i will be in athens next weeks.
dont trust falcon manos in athens to mutch.