But be aware that when selecting "737-300" from the menu, the software assumes 22K of thrust. The IXEG 737 models the one with 20K engines. So in TOPER, I select the 737-500, because the 500 in TOPER is based on the 20K engines. Even though not entirely correct, performancewise in the sim, I would say it makes little difference to choose another airframe.
On the other hand, try to enter an airport with a fairly short runway (around 2000 meters or so), and TOPER will warn you if you try to enter near or at maximum takeoff weight (approx 63.2 tonnes for the 300). I also tried reducing the weight a little bit (still close to structural MTOW), and TOPER suggested around 33 degrees C assumed temperature.
I did one test at ENHD (Haugesund airport, on the coast of southwestern Norway). The runway is approx 2000 meters, I topped the plane with payload and fuel so that the takeoff weight was around 58 tonnes, which was the highest weight TOPER allowed me to have with an assumed temperature of 33 degrees. I cannot remember the wind, but the OAT was around 20 degrees I believe. Flaps was 5 degrees (TOPER does not provide calculations for flaps 1 takeoffs).
I then used X-plane's default failure system to initate engine failure just prior to V1. When the engine failed, I immediately retarded the thrust levers to idle. The autobrake kicked in with RTO and slowed me down to a stop just a tad before the runway end, but still with some meters to spare. Note that I did not use ANY reverse thrust at all, and no speedbrakes either (to put extra weight on the main wheels). Still, it worked out fine. I would say, that if TOPER was giving me the wrong data, I would probably have made my way into the geography beyond the runway. I'm also pretty sure that the accelerate-stop-distance (that is, the distance the plane would use to accelerate to V1, experience an engine failure, and then come to a complete stop), is purely based on brakes only, and not taking thrust reverse into account.
I also did the experiment on other shorter runways, and coming to a stop after an engine failure near V1 was perfectly manageable using RTO on autobrakes only. I encourage you to experiment a little bit with this. And don't choose the longest runways, because here, the structural takeoff weight will always be limiting, no matter what.
One limitation I can see with TOPER so far, is that it always wants to calculate the highest possible assumed temperature for any given runway, wind, OAT, pressure and weight. For instance, the program might warn me if I want to takeoff from a 2000 meter runway with 61 tonnes weight using an assumed temperature of 30 degrees C or more. But, taking of with full thrust from the same runway might work fine. At least, that's my opinion so far.
There are a number of factors that determine the maximum allowable assumed temperature, as well as the maximum allowable takeoff weight for that assumed temperature. One good place to read about assumed temperature is the B737 Techincal Site (maintained by Captain Chris Brady of Easyjet).
Also, the difference between assumed temperature and de-rates are also well explained.