According to the ineligible products list, the Boss and the GT500 are no eligible for X-Plan pricing.
The formula for X-Plan pricing is:
X-Plan is a pre-determined price based on dealer invoice or A-Plan price depending on brand. The X-Plan price is noted on the dealer invoice.
Dealer Invoice - (0.4% * Dealer Invoice) + Administration Fee*
Dealer Invoice + (0.6% * Dealer Invoice) + Administration Fee*