Globally define kerning of math letter

Question

Is it possible to have a kerning reconfiguration in the preamble to allow a small amount of whitespace for specific letters.

In this example, I would want right space to the right of g and f when used as an italic in an equation.

Minimal Example, compiled using xelatex

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathspec}
\setallmainfonts{Adobe Minion Pro}
\usepackage{MnSymbol}
\let\mathdollar\relax
\usepackage{lipsum}  

\begin{document}
\lipsum[1]
\begin{equation*}
    \begin{aligned}
        \delta n_a &= \delta \left( \frac{\epsilon \partial_a f}{ \sqrt{g^{bc} \partial_b f \partial_c f }} \right) \\
        &= -\frac{1}{2} n_\mu \delta g^{bc} g_{bc} \\
        &= \frac{1}{2} \epsilon n_a n^b n^c \delta g_{ab} \qquad \qquad \epsilon n_a n^c = g^c_a - \gamma^c_a \\
        &= \frac{1}{2} n^b \delta g_{ab} + c_b \qquad \qquad c_b = -\frac{1}{2} \gamma^c_a \delta g_{bc} n^b 
    \end{aligned}
\end{equation*}
\lipsum[1]
\end{document}

Screenshot as an example

Edit

Included regular text around screenshot to show kerning of body text and included code of a minimal example

Answer 1

There's no automatic way, sorry. The author of mathspec explains why. The reason is that basically mathspec is a kludge.

You can use " in appropriate places, though. For more information look in the manual.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathspec}
\setallmainfonts{Minion Pro}
\usepackage{MnSymbol}
\let\mathdollar\relax
\usepackage{lipsum}  

\begin{document}
\lipsum[1]
\begin{equation*}
\begin{aligned}
\delta n_a
&= \delta \left(
     \frac{\epsilon \partial_a "f}{ \sqrt{"g^{bc} \partial_b "f \partial_c "f }}
   \right) \\
&= -\frac{1}{2} n_\mu \delta "g^{bc} g_{bc} \\
&= \frac{1}{2} \epsilon n_a n^b n^c \delta g_{ab}
   && \epsilon n_a n^c = g^c_a - \gamma^c_a \\
&= \frac{1}{2} n^b \delta g_{ab} + c_b
   && c_b = -\frac{1}{2} \gamma^c_a \delta g_{bc} n^b 
\end{aligned}
\end{equation*}
\lipsum[1]
\end{document}