Here's what I've got:
For $x\in [0,\pi/n]$, we have $\sin(x)=x(1+O(1/n^2))$. Similarly, we have $\sin^n(x)=x^n(1+O(1/n))$. Let us write $f(x)=\frac{\sin(x)}{x}$. We have $|f(x)|\leq 1$, but for any $\epsilon>0$, $\exists \delta>0$ such that $f(x)\geq 1-\epsilon$ for $x\in [0,\delta]$
Then, for $x\in[0,\pi/n]$: $ \prod_{k=1}^n\frac{\sin(kx)}{\sin(x)}=n!(1+O(1/n))\prod_{k=1}^n f(kx) $
Now, we have \begin{eqnarray*} J(n)&=&\frac{2}{\pi}n!(1+O(1/n))\int_0^{\pi/n}\prod_{k=1}^n f(kx)\,dx\\ &\geq&n!(1+O(1/n))\int_0^{\delta/n}(1-\epsilon)^n\,dx\\ &\geq&\frac{2}{\pi}(n-1)!(\delta+O(1/n))(1-\epsilon)^n \end{eqnarray*}
Similarly, we have $ J(n)\leq 2(n-1)!(1+O(1/n)) $
We can combine these two estimates to get $ J(n)=(n-1)!(1+o(1))^n $
Of course, $(n-1)!=n!(1+o(1))^n$, so we can write $ J(n)=n!(1+o(1))^n $ It might be useful to use Stirling's formula to get $ J(n)=\left(\frac{n}{e}(1+o(1))\right)^n $
Using Mathematic, I computed $\left(\frac{J(500)}{499!}\right)^{1/500}=.99554\ldots$