Yes, it is possible on E115 too. You can overclock it to 168 Mhz, I think. To do that you actually have to open the unit and do some soldering and/or use graphite pen for connecting two open connectors.

You have all instructions on
http://www.brighthand.com/html/howto/overclock1.html

> WHOA! Overclocking?! Is this possible on the E-115? How did you do it?
>
> > Speed difference is quite big if you overclock it to 180MHz. About 15% I'd
> say.
> > It's particulary obvious in CB64 and Pocket NES because they do not run full
> > speed on normal 150MHz Casio. Unfortunately, at 180MHz dreaded MMC slowdown
> > problem is quite often. Check the brighthand discussions for more comments
> about
> > this. I ended up overclocking it at 168MHz. Everything works perfect, slowdown
> > never happens, and the speed increase is about 10% which is also not bad. You
> > may want to wait a bit more because Jimmy software should be releasing
> software
> > overclocker for Casio just like they did for the iPAQ.
> >
> > > I've seen on other groups that you have overclocked your EM500. I was
> > wondering
> > > if you have seen any appreciable speed differences in using various
> emulators.
> >
> > > I have been fairly happy with the speed of Mameboy, but other emulators
> > > (ComeBack64, and the NES emulators in particular) are just too slow to be of
> > any
> > > real use to me. I have thought about taking my EM500 to 180 MHz, but I
> don't
> > > want to risk damaging the unit if the speed increase is negligable.
> > >
> >
>