Implemented optional overclocking via pcv in the latest commit. Requires libnx 1.6.0 to build now.
The "overclock level" is changed via the nx_overclock variable in dhewm3.cfg:
0 (default) - no overclock
1 - CPU 1224 MHz / GPU 691.2 MHz / MEM 1600 MHz
2 - CPU 1683 MHz / GPU 768 MHz / MEM 1600 MHz
3 - CPU 1785 MHz / GPU 768 MHz / MEM 1600 MHz
Later two levels make it almost playable even with shadows on. Not sure whether these clocks are completely correct and safe for prolonged use, so use with caution. Did play for about 20 minutes with no ill effects on level 3 though.
New release which includes this change and others will probably come soon.