Whilst I agree that the XP to DP expenditure seems excessive at high levels it is well balanced and realistic.
The higher levels is where the non-spell user expend their DP's to learn complementry skills (Combat Manuevers and Special Attacks)... however a direct increase in DP will still favour the spell using classes.
I personally like the idea of training to improve stats past their potientals.. especially for higher level characters, this has the effect of increasing stats, and more importantly the stat bonuses. This I allow as part of the normal stat gain roll, however, the rationale could be made for DP spent dedicating yourself to further improvement...
This option is already in the rules anyway. 8DP for a additional Stat gain roll. However, at high levels most characters will have maxed out. (usually by 5-7th level)
I allow a system which allows the increase of stats past their potiental (effectively increaseing the potiental).
This relates the Current/Potiental stat to the results of the stat gain dice but only when the results are doubles. (Normally you add the total with results of 5,5.. 6,6....7,7 and add them to the current...up to the maximum potiental).
I allow this potiental to be increased slightly if the number rolled (a double ie. 66,77,88,99,100) is equal or higher than 10 less than the potiental. The actual amount the Potiental is increased by is based on the following formula:
(Stat Gain dice result/2)+2 -(potiental/10 round down) = increase in potiental.
Any result of 00, will result in at least a 1 point increase regardless of the Potiental.
i.e. a potiental of 75, can be increased with rolls of 6+6 (1pt), 7+7(2pts), 8+8(3pts), 9+9(4pts) and 10+10(5pts)
e.g. a character with a current of 75 and a potiental of 90, rolls an 8+8. This automatically increases the current to 75+10 = 85. It also increases the potiental to 91.
This could be expanded by decreasing the cost for additional rolls made when the Current = Potiental.
It is obviously a risky proposition and unlikely to gain automatic success. I'd say a character could only make one such attempt per stat per level increase. (You could also bias the DP cost for certain stats and also decreasing the costs for Arms users and increasing them for Pure Spell Users).