The only thing really different is the drive links are thinned down where they stick into the bar, unless you are thinking of pixel or nk which only comes in 0.050. Around here I think 0.058 is what is most likely it should have a bit better wear resistance at the sprocket I would think.I'd like to have 18" bar but would consider a 16". What gauge chain is better .50 or .58 on these saws? .50 seems like it would cut faster being lighter ?
There are threads on here to search up about the internal differences and you can study the parts sheets. And if the price premium is worth it. I think the main thing is the crank fully or nearly fills the crank cavity on the 550 while the 545 has the more normal shape perhaps half filling the crank cavity. The more power ones generally make more noise and vibrate more from the spec sheets.