The execution speed of a C++ executable is only insignificantly faster than that of a Delphi executable. But think of all the design-time advantages you have if you choose Delphi. Delphi has the fastest compiler on the planet, it has full DirectX and OpenGL support through the JEDI headers, it supports assembler...

There's really no reason why you shouldn't be able to write a commercial quality game in Delphi. The only downside is portability. Delphi apps can easily be ported to Linux (through Kylix) but porting a Delphi game to consoles is a little tricky. Other than that, go for it! There are tons of examples of great 3D games written in Delphi (Fist of Benztown, Pythian Project, etc.).