In the case of employees whose F contributing salary is already Rs 15000, any addition to special allowance will not make any difference. For example, Your total Salary is Rs 150,000 and your Basic pay on which PF is deducted is Rs 50000. That means Rs 6000 is your employer's contribution included in the CTC, ie, Rs 156000 per month. Now if the employer reduces it to 12% of 15000, then the share will be Rs 1800.Since the CTC of Rs 156000 is already agreed with you, the difference of Rs 4200 will be paid to you by the employer and this can be paid to you under any head because this will not attract PF because the PF is contributed on Rs 15000.
Now it is not necessary that the employer can only contribute on Rs 15000, he can contribute it on any amount and for that there is no limit, in fact.