I've done a run against Ascot and Belmont from the start of 1998 to the present.

Using the exact system you put forward:
1086 bets, 171 wins, 896 units returned for a 17.5% loss.

Forget the barrier constraint:
1506 bets, 245 wins, 1351 units returned for a 10.28% loss.

Use 6-4 or better instead of 2-1 or better:
1675 bets, 306 wins, 1513 units returned for 9.66% loss.
A little better and a few more bets.

I tried tweaking the other factors without any success except for the raced within 15 days rule. As you tighted the requirement down towards raced within 8 days the return improves to a slight profit but the number of bets falls off a lot making it a long wait between bets.

These figures are based on the SP at the jump so if you can snag anything more than 10 percent better prices any time before the race you will be coming out ahead.
Sorry I didn't test it with the staking plan - too much programming too little time.

Hope this helps.

K V