First, Texas did not join the Union by treaty. It joined by joint resolution of Congress. According to Dr. Felix D. Almaraz, a professor of Texas history at the University of Texas, San Antonio, DeLay is correct that Texas can, in theory, divide into as many as four additional states (five total) and has tried and failed to do so from time to time. But if it did, it would result in eight (not 10) senators, and they wouldn't be from Texas, except, perhaps, by birth. They'd be the two senators from each of the (up to) four new states.
But there's a problem with that. According to Article IV, Section 3 of the Constitution, "[n]ew states may be admitted by the Congress into this union; but no new states shall be formed or erected within the jurisdiction of any other state; nor any state be formed by the junction of two or more states, or parts of states, without the consent of the legislatures of the states concerned as well as of the Congress."
But even if Congress couldn't vote down the proposition, Steven Teles, a professor of public policy at the University of Maryland who studies federalism, says that, to the best of his knowledge, "[t]here is no provision in the U.S. Constitution for ejecting a state from the Union, just as there is no provision for secession. Becoming a state, from the point of view of the Constitution, is a one-time-and-one-time-only affair."
That's not to say there's no other hypothetical version of Texas secession that isn't at odds with the law--Texas is a Very Mysterious Place. It's just that DeLay's idea is a bit fantastical. Perhaps the solution lies with this guy, whose wide-ranging powers are famous both in politics and the world beyond. If Chuck makes it happen, or even if Texas secedes in some other way, the young Republic will have a hard time finding somebody to run against him in its first presidential race.