The slow discharge under full load is, in my opinion, totally fine. The alternative that I do not want to see is to cripple the laptop's performance.
However, I also call on you, notebook check, to test for this behavior with a third-party charger of a higher rating, like the 87W Apple charger or, if you can find one, a 100W brick.
If I were to buy this laptop, I would want the MX150 version, yet I would not expect to game for many hours at a time on such a system. Therefore, the presumably smaller, lighter 65W charger is a better value for my use case. I would enjoy both portability and high burst performance.
Please don't encourage Razer to cripple their laptop's performance with a firmware update. I see this in my Dell XPS 15 9560. When charging over USB type-C it will draw half as much power as with the stock barrel charger. In order not to discharge under load, the GPU performance is crippled, even worse than running on battery! I strongly regret this "feature" which, in my opinion, serves only to avoid the kind of public complaint you have raised.