Frontier markets are storming higher, extending last year's stellar performance and outpacing emerging market peers as risk-seeking investors pile in, but their performance has raised questions over whether the exotic asset class is getting ahead of itself.

The MSCI Frontier Index's 20 percent year-to-date rise follows last year's 22 percent rally. Its gains dwarf those of the MSCI Emerging Markets Index, which is up 6 percent year to date.