Now that you have an iPad jailbreak, you have many more options and customizations at your fingertips. RetinaPad and FullForce both take advantage of your iPad screen size for iPhone-only Apps. The Apple standard practices are silly in placing a smaller iPhone app on your iPad (320 x 480), especially since the iPhone display was upgraded to retina a few months following the release of the original iPad.
RetinaPad will display the retina iPhone version of an app in it's full resolution. Why this is not the default 2x mode is mind boggling. By default, the 2x mode pixel doubles the low res iPhone version to make it look bigger. FullForce forces an iPhone app to use the iPad's fullscreen native resolution, instead of confining it to the small rectangle in the centre of the iPad screen.
Some applications do not work properly with FullForce. In these cases, RetinaPad is usually always better than the standard 2x mode. It is much easier if an iPad app version is released, but more often than not...the iPhone versions are more familiar and used much more often.