In assessing the performance from the .72 engine, I would suggest this as being the optimum size for the Phantom. It’s powerful enough and light enough to limit the additional weight needed to balance the aircraft. Installing a heavier .91, plus the additional ballast that you’d need, would, I’m sure, take the shine off the sparkle. If there has to be a cautionary note I’d say this isn’t a model for small, restrictive flying sites. It’s a big ‘jet’ (almost 6’ long), so give it the space it deserves and you’ll love it.