Yeah I'd say it's a bit high, maybe drop it down to the same br as the spit 2b or even 2.7.
for your problems there could have been new aircraft introduced or old being moved around into the 3.3-4.3 bracket, if your feeling a bit rusty (not meant as an insult), I find having a few matches at tier 1 can usually help with that.
As for tips for the boomerang, the engine is good at lower alts, around 3000m is where I can get the most out of it before the performance starts dropping. Try to stay near allies or jump into dogfights that are in progress to shoot at a tunnel visioned enemy plane, this can help you and your team. If your not sure you can hit the target, fire the mgs, leave the cannons for shots you know will hit or for targets you need them to take down quickly. Know what threats you could be facing, if you keep being killed by a specific aircraft, look at tutorials for that plane, they may show a weakness.
dont go near a bv238 unless absolutely nessisary, if possible try for frontal pilot snipe from below (avoid the top turret), long range engine fire or focus fire on 1 wing tip.