I ran two simulations to answer your query: in one, the player forgoes X-X splits; in the other, he splits as appropriate against 5's and 6's.
Your post was missing several key factors, so I guessed the following. You're using what Norm in CVData calls "Basic High-Low". The game is SD, H17, DA2, No DAS, No RSA, with a penetration of rule of 6 and one other player. In calculating the True Count, you estimate the remaining cards to the nearest quarter-deck, and you bet $5*TC for positive TC's up to a maximum of $20, and $5 for all neutral and negative TC's. Note, as Don Schlesinger indicates in BJA3, that for HiLo with quarter-deck resolution you will never see TC's of exactly +3, +7, +11, ...; nor of -7, -11, -15, ... Therefore, you will never bet $15 under this scheme.
Now for the results!
With no X-X splits, your IBA is -0.692% and your TBA is -0.624%, so with a total action of $869.28/hr, your win rate is -$5.42/hr.
If you include X-X splits against 5's and 6's (both at +5), then your IBA is -0.631% and your TBA is -0.567%, so with a total action of $873.33/hr, your win rate is -$4.95/hr.
Thus, you can clearly see that including X-X splits in no way compensates for the short-pay of the crapjack game.
By the way, these spreads are not nearly optimal. In either case you first see an advantage at +4. Your edge first exceeds +2% at a TC of +9 (without X-X splits) and at a TC of +8 (with X-X splits).
Hope this helps!
P.S. I will admit to having intentionally played crapjack myself. Many years ago, the old Treasure Bay in Biloxi used to deal a 6:5 SD game but with several compensating factors: it was S17, DA2, DAS, and was dealt to the last card. The dealer would shuffle and, after the cut, burn one card. He would continue dealing until he reached the 52nd card, which he would also not deal. If he ran out in the middle of a round, he would simply burn the last card, shuffle the discards, and after the cut burn one card and continue the round. Between the possibility of "end play" and my 1:15 spread, I had no trouble beating this game.