You are asking two different questions: what would be the model result, and what would be the actual performance of an actual portfolio.
The optimal model results with the S&P 1500 will be at least as good as the model results with the S&P 500. The S&P is a proper subset of the S&P 1500, so you can get the results of the S&P 500 model by solving the S&P 1500 model with the added constraints that all non-S&P 500 shares have zero exposure.
More explicitly if you partition the exposure variables into $x_{500}$ and $x_{1500}$ (where $x_{1500}$ represent exposures to S&P 1500 \ S&P 500) and let $y$ be a vector of all other variables in your model, the two optimizations are
$$\eqalign z_{1500} = {\rm maximize} f(x) \hspace{0.2in} \\
\mbox{subject to:} {\hspace 1.5 in} \\
g_j(x_{500}, x_{1500}, y) = b_j {\hspace 0.15in} \forall j$$
and,
$$\eqalign z_{1500} = {\rm maximize} f(x) \hspace{0.2in} \\
\mbox{subject to:} {\hspace 1.5 in} \\
g_j(x_{500}, x_{1500}, y) = b_j {\hspace 0.15in} \forall j \\
x_{1500} = 0 {\hspace 1.38in} $$
For optimal solutions to both problems $z_{500} \le z_{1500}$.
If you are using a heuristic to solve both models instead of using a MIP solver like gurobi or CPLEX, then it is theoretically possible that your S&P 500 model will produce better results. In fact, that might happen if, for example, you solver is running with a restrictive time limit, you have cardinality and minimum investment constraints and most of the optimal exposures happen to be in the S&P 500. Even with a MIP solver, you are solving to a tolerance. So if you are using the default MIP tolerances
$z_{500} \le z_{1500} \cdot (1-{\rm MIPGap})$.
As far as actual performance of the two portfolios, if your model has some validity to the real world (which is questionable), then you would expect the S&P 1500 portfolio to at least match either the return or variability of the S&P 500 portfolio. I say expect because even portfolio optimization theory doesn't say that a given sub-optimal portfolio will under-perform an optimal portfolio over any single time period.