I don't think he (or she?) even considered using the high performance stuff. As the story has told thus far, even if he's able to acquire currency to use in the cash shop, it's not easily obtainable (even with his current relationship that can somehow provide a stable income), and the cash shop currency is required for clips, and probably other basic stuff like "fuel" or whatever, so using the big guns would be a waste, it's obviously much better to use the most suitable equipment for a current job keeping the cost as low as possible, and steadily accumulate cash shop currency for emergency situations. We also don't know if the stuff in the cash shop is readily available upon purchase, or if it takes time to equip, like he apparenly seems to hint at when he said he "prepared" for such a scenario with the current anti-air rifle, so switching to the stuff in the middle of a harpy swarm doesn't seem like the smartest idea, and given the sudden giant burd event, I doubt he had the presence of mind to react in a split second to save the girl. All in all, it makes sense, but it could have been told much better, at least I believe so.