I have in the past used two different Verizon Westell modems. Westell is well regarded in terms of ADSL signal performance as having one of the best chipsets out there (everything else about them is uninteresting). The newer modem delivered noticeably better signal performance than the older modem.
Please note the difference between signal performance and end-user visible throughput. If you have enough signal headroom -- that is, your actual SNR/BER/etc. signal performance is significantly greater than what is required to reliably recover a signal at your provisioned data rate -- then what modem you have doesn't affect your delivered IP throughput, you get exactly the same result. Where it matters is when you have less headroom (you're close to the limits of what your physical line/plant can deliver), or when you have intermittent or flaky conditions (e.g., weather changes the performance of the lines for the worse). If a better chipset can eke out, say, 1-2dB of better signal, that can be the difference between visible loss or delay, and smooth sailing. A little bit of end-user visible loss, or even delay due to retransmission, can cause very end-user visible throughput problems and web-page loading problems.
Telcos *usually* are quite conservative about what data rate they will provision on a DSL line, so that under normal circumstances you'll have enough headroom, period. So basically, the conditions under which the modem's signal performance matters should be rare. But it's exactly those intermittent, rare cases that are also really hard to diagnose and fix, and if a better modem chipset can just make that problem go away, it's totally worth it!