Thanks for starting this thread, as the varied replies were really interesting to read. Specific to your first question, any cue is only as good as the player using it. To that point, I enjoyed the thread reply that highlighted Efron Reyes title win with a $14 cue using an Elkmaster tip (noting Efron "soaks" those tips overnight to soften the tip).
So with that thought in mind, if one is intent on improving play with a cue upgrade, you need to determine what your budget will allow, and then explore the range of options (vintage/antique cues, production cues and custom cues). From a historical context, many great cue makers (Rambow, Balabushka, Paradise, Szamboti, and others) utilized Brunswick Titlist factory blanks, with Rambow distinguishing himself by using Titlist Professional blanks during the 1940's (which is has been my main playing cue for many years). Buying any of the above makers these days runs into the thousands, though there are sometimes opportunities to buy Brunswick "Willie Hoppe" inscribed cues in the $300-$500 range that were made from that same Titlist stock. If you can locate one in good condition, my experience is that many of these cues not only hit very well indeed, but also have a stable aftermarket value that's likely to increase in the years to come.
In the realm of product cues, one of the other posts referenced McDermott cues. I would respectfully disagree relative to the modern low end cues made by McDermott, as the stock is prone to warp (especially the "Lucky" line up) and frankly represents poor value in the sub-200 range. That said, either a vintage or good quality modern McDermott cue plays relatively well, and tend to hold their value in the aftermarket. To that point, I still own the first McDermott cue purchased when I was teenager working/playing in a pool room on a near 24/7 basis. While well worn and battle scared, the Birdseye Maple that cue is made of is just as straight now as the first day I owned it, (I'm in my sixties now).
Further, in the $100-$500 range, I find the Players PureX / HXT line up of cues to represent the best value in that space, as this approach gives experience with use of a decent quality Low Deflection shaft.
In the $200-$400, I find Schmelke cues represent solid value. Consider the
SPL Full Spliced cues and the
Schmelke CC220 Series to represent good value and a greater degree of customization, especially when paired with low deflection shafts. I would recommend selecting a 3/8x10 pin with flat faced joint as this will allow you select from the widest range of low deflection shafts in the aftermarket. I personally use HXT, G-Core and Predator with my Schmelke.
In the $500+ area, the options in both production and custom made cues are nearly endless. My personal favorites in the production area Pechauer, Shon and Joss West. I would stay away from Meucci, as there's often a high degree of variation in quality control. I would defer to other folks posting here specific to the modern custom cue makers in the market. Hope this is helpful.