In response to the question, the standard belief is that a goy is only better off converting if he is going to be observant. Why would he convert anyway if he only plans to be observant for a little while? If that’s really the case, it’s questionable whether the conversion is valid — conversion involves a lifelong commitment.
Then again, historically people were converted before they even knew what most of the mitzvot were. Following that logic, some LWMO rabbis today (such as Rabbi Marc Angel, and I believe Rabbi Joseph Telushkin) advocate for allowing people to convert (especially if they’re already married to a Jew) without completely accepting the mitzvot. Of course that’s controversial, for good reason!
A couple other comments though.
First, I’ve heard people say that goyim only have a share in Olam Haba if they are ben Noach. But does anyone know the source of that? I’d be surprised if that’s the only view. If “all nations have a share in the world to come” and G-d is kind and merciful and everything, how could He let the vast majority of the world (those who aren’t bnei noach in the strict sense) just die with no afterlife? It doesn’t make any sense.
Second, all this talk about whether someone merits olam haba or not is not the only way to look at things. Belief in gilgulim is not just for chassidim and mekubalim — even the Gra wrote a sefer (about Yona HaNavi) full of stuff about reincarnation. Also, we don’t believe in eternal hell, right? Basically everybody will either get purgatory (for a year or less), olam haba right away, or get another gilgul — right? In truth, though, it can be more complicated, since the Arizal said we actually have more than one soul, and parts of various other people’s souls, so it’s a little complicated to talk about what happens to each individual after death.