I hate to ask, but have you guys flown a Super Tigre .45,
and a Evo .46,
and a SIG .53?
I have. You'd be amazed how much free time in a day you have eliminating women from your life!
I didn't want to guess what outcome I'd get, so I enjoyed testing engines for years on my planes. I like trying engines with the fast clamp mount from Great Planes on my low wing planes. There are a lot of surprising results.

Each engine performs differently on prop, fuel, glow plug, and airplane it's bolted to. Trainers have drag, and the wing is mounted with 10-12 rubber bands. It's a flat bottom wing? It's designed with dihedral, and I wouldn't want to push the plane too hard in that configuration.

The SIG is utter garbage, one of the worst engines I've ever come across. It ran hot on everything. It was sent back and replaced, the carb is a joke, the tuning was nearly impossible to pull off the first time, and I've tuned hundreds of other engines over a few decades. It's make Tower Hobbies look like a Rossi, and Tower isn't high on my list either in quality and consistency. One is great, the next is a dog.

The Tigre in fact is much more powerful than the Evo, considerably lighter, and swings a bigger prop, on less nitro content fuel. My suggestion is dumping the Evo muffler, getting a header and pipe, or mousse can and wake it up that way, and since it's covered in fins, throw some more nitro content at it. Start using APC props. Evo mufflers are very restrictive. Most of my OS engines I use the GMS .47 ot Tower Hobbies Pro .46 muffler, $14, good increase in power for the money, it will also fit Magnum, Evo, ASP, and other OS clones.

There is no gain on a .51 Super Tigre over the .45. The trainer version of the Evo is toned down a lot from the NT, and the drive washer has that heavy balancer on it, I suppose to help idle.

Speculating one engine over another based upon displacement is not sound advice or practice to follow. Reading specs on a owners manual is even more misleading. I love reading the 61 FX HP is 1.9 at 16,000 rpm. You'll never hit 16k! My Rossi .45 hits it at 16,000 on the ground, and tunes just as well. It was made to run 16,000. Do you think you can hit 16000 on the ground with a .46 or .55 AX? How many flights do you think it's good for at that rpm?

Flying these engines really proved a surprise to many. I do not consider any OS AX of any displacement a 'fast' engine other than the .25FX, that truly is a screamer, and I assume the new .35 AX is, and the old .32 SX was. But in the .40 - .50 class and above, they really are just sport engines, which is fine for any trainer. It's a great tuning engine, all novices with limited tuning skills love them as they should. The carbs are great. But it's not a performance engine, relatively speaking. I generally fly the older Super Tigres from Italy, round billet head, they just run great for me. But for speed, Dub Jet, Nelson, Rossi, Webra, MVVS, HP, and other high performance engines really make OS just another sport engine.

Try 9X7 prop, 15% nitro fuel, replace the muffler with a Tower Hobbies Pro .46 muffler. You'll be surprised what a few changes will do to it. For $159 for an AX .55, I'll take the Rossi challenge any day.