A 1:8 twist won't negatively affect performance with lighter bullets under casual use. But if you're looking for the ultimate small group with one particular bullet, it'll require some compromise in perfection with another bullet. A faster-than-needed twist won't take a shorter bullet out of the X-ring, and there is no getting a longer bullet to shoot through a twist that's too slow. You have to get the twist rate that will stabilize the longest bullet you'll shoot and live with that. But you'll probably never notice your compromise.
Free-float the barrel you choose to take stress off it, or make sure a float tube is in the rifle package you select. Float tubes come in a myriad of styles, lengths and more, but they all work the same. In essence a float tube is a pipe that replaces the barrel nut. The barrel nut holds the barrel (barrel extension) into the upper receiver. With a float tube installed, nothing bears against the fore-end (which is the float tube).
There are many different takes on float tubes. Tubes are made to conform a rifle to specific applications, including the sneaky Service Rifle "undertubes" that float a barrel under a set of standard handguards. There are flat-bottomed fore-ends for bench shooters, extra long tubes with accessory rails for High Power shooters, carbon fiber units for varmint hunters and so on.
The Gas System
No trick gas system will make an AR-15 shoot better. The only influence the system has on accuracy comes when stress exists that isn't supposed to be there, and that can come from a binding gas tube. If the gas tube is binding, it has to be bent or, rather, "inclined away from contact" because the little stainless steel tube will fold over without warning. Check for stressful contact by removing the bolt from the carrier, putting the carrier into the upper and sliding the carrier key in and out of contact with the tube. It helps if you can get a finger on the gas tube, but watch and listen for excessive contact that will have the tube moving the carrier. Contact is necessary, but binding is not. Look also to see that the gas tube is floating in the center of its receiver hole.