Long winded answer: "Does XL2540 or XL2720 have less lag during blur reduction?"
I suspect XL2540 can have less lag, but "it depends" on whether NVIDIA ULMB correctly works with large vertical totals.
For blur reduction, the best way to combine eSports (lag reducing) & Blur Reduction (lag increasing) you want to find the correct special sauce -- (1) NVIDIA's ULMB, or (2) BENQ Blur Reduction AND you also want a monitor capable of large Vertical Total support. The use of large Vertical Totals helps reduce input lag of specific refresh rates (with or without strobing), which can compensate for the increased lag of blur reduction (typically 1/2 of a frame extra lag)
This doesn't matter if you're using the highest refresh rate (which requires reduced totals), but blur reduction strobing usually requires a lower refresh rate than the monitor's maximum. For example, several 240Hz monitor only supports strobing up to 144Hz. In this particular instance, you want a monitor that can accept a large Vertical Total, to help deliver the refresh cycles faster from the computer to the monitor.
A monitor that can embed a 1920x1080 signal at the very top edge of a ~1920x2000 signal (e.g. VT2200) means the first 1080 lines will be delivered from the computer to the display twice as fast (4ms less input lag for bottom edge of screen, while staying at the same 120Hz refresh rate). So you've got a faster actual display scanout and a longer pause between refresh cycles (before the next scanout). This benefits strobing in two ways (1) lower latency, and (2) clearer motion, because you've given more time for LCD GtG transitions to finish before flashing a fully-refreshed refresh cycle (example of a blur reduction strobe backlight:
http://www.blurbusters.com/lightboost/video ... All of them, including ULMB, works in essentially the same way).
Blur reduction can add extra input lag (because you're waiting for a display to finish refreshing in total darkness, before flashing the whole screen) but there are many tricks to reduce input lag of blur reduction -- including the large vertical total trick. I have not extensively tested blur reduction of the new XL2540's to make any comments about it yet. That said, of the previous generation (144Hz, not 240Hz), the most flexibly-adjustable blur reduction monitor is the BENQ/Zowie monitors (XL2411, XL2420, XL2720).
Ideally, NVIDIA's ULMB should be large vertical total compatible, but it isn't always. (ULMB does automatic strobe phase adjustment on its own, so there's no need to adjust timing of strobe -- but ULMB should still benefit from Large Vertical Totals)
As I have not tested XL2540 yet, I'm not at a position to say if XL2540 has lower-lag strobing than XL2720Z. But if ULMB works perfectly during large vertical totals (and adjusts strobe phase to compensate for faster scanout), ULMB should automatically have less latency during large vertical totals. This will need to be tested-for.
But if done well, if you deliver a 120Hz refresh cycle in just 1/240sec (double-size vertical total), you can reduce the input lag penalty of strobing from an average of 1/2 frame to a mere average of ~1/4 frame (2ms penalty). This reduction of "LightBoost lag" to a scant mere 2ms can actually make strobing sufficiently eSports-friendly to some people, if the competitive advantages of blur reduction more than makes up for the 2ms extra lag. More testing will be needed...