I have the motion down for both of these techniques but I cant seem to speed them up for faster death core and metal drumming, is it just down to physical strength of the forearms that determine the speed or is the a way to alter my technique to make it bounce faster on the snare. I also think that my basts and gravs are far too quiet how can i increase the power of the hit?
In my humble opinion, there are two things you need to separate:
Physical strength plays very little role (up to the insane speeds that death core dudes operate) on your blast beats and almost zero role on the one handed rolls.
Forearms plays a big role but your whole arm is involved hence takes a part in the overall technique and again almost zero role on the one handed role.
Pillow practice is not liked here for some reason (See Drum set made from towels/pillows). But for the blast beats the main trick is avoiding flams at higher speeds (weirdly when you practice flams you avoid unison playing, brain is funny). The main issue on the real drumset is that the bounce of the cymbal is different than of the snare so you need to teach your brain to compansate it on the fly. The easiest is to play one hand on pillow one on the practice pad. And pillow at those speeds is tough to play so don't force it immediately just try to get a decent coordination and practice later on real drums too (also alternate hands).
For the one handed roll, you need to increase your technique either via Moeller motion or by stick control via finger sticking. I recommend the former as you can do other nasty things with it as a bonus. You should not try to have action from forearm that would temporarily increase your speed but your tempo and consistency will detoriate over time. Instead it should be more of a slight of hand in a natural fashion. Google Johnny Rabb for extreme use of it and how the stick moves inside in his hand (you don't need to copy it but it's a good example for economy motion).
For the quietness, it is because of your hand technique and level. Your brain is still trying to figure out how to tense up the forearm and release it at those speeds because you are not used to the economy motion using different techniques such as Moeller. Hence in nerdy terms your bandwidth is not large enough to focus on muscle tension. Think of your feet, until you have nailed the single strokes you were busy keeping them in tempo now you can concentrate more on how to contract groups of muscles without losing the tempo to speed up. One handed rolls gets faster once capture the essence of hiding the rim sound inside the actual snare sound then by increasing your stick travel gradually you can build up speed but for that again you need a proper stick control framework that you are comfortably progressing. In general one handed roll doesn't get super loud due to the bounce stroke limitation.
EDIT: Found it. Watch the very end for the mind-boggling part. If that doesn't cut it, stop playing drums :P I really really really recommend buying his DVD Secret Weapons
Increasing speed for technique is something all musicians strive for with almost all technique, and the same principles apply across all instruments.
First, you need a metronome - plain and simple.
Second, a common technique is to move up two clicks on the metronome and then back down one step. You will need to decide on a length of time to play at that tempo - say two measures. Make a goal to be able to play the two-measure segment 10 times correctly (continuously!) before changing the tempo. Start at 60bpm and slowly increase over time - weeks and weeks.
Fast technique is slow technique done in a short amount of time. If you want to develop fast technique, you need to develop excellent slow technique to develop fine motor control necessary for fast technique.
For percussion, dynamics are a function of speed, not power. If you want to strike the drum louder, you merely need to strike the drum more quickly. Be careful that you are not tense. Also be aware that due to the type of music you play, there is going to be an apex between speed and volume - you're only going to eventually be able to play so loud and so fast, so just be aware.