Hello Greg. Welcome to the board, and thank you for your interest in the XTB-IIR. Also, thanks to those that answered before me.
To answer your questions:
1) The XTB-IIR will certainly help with weak signals. However, excessive powerline noise can directly prevent receiving modules from decoding commands. The XTB-IIR may not help with powerline noise unless the receiving modules incorporate some form of AGC to raise their detection threshold above the background noise level. Most Leviton devices do incorporate gated AGC, and they do quite well handling powerline noise. Possibly they have increased their detection threshold high enough that they are not reliably receiving signals from your CM15A anymore.
FYI: I understand the ESM1 has been discontinued, as has the TesterLinc. I'm considering offering my own tester to fill the gap.
2) While the XTB-IIR is best installed close to the distribution panel to minimize loss in that run, a 20ft run to the dryer receptacle should not impact signal levels that much.
3) There is nothing particularly difficult about assembling the XTB-IIR. There are no surface mount devices. You will need a good magnifier to read some of the part numbers. A lighted bench magnifier works very well. Like someone else said, read over the assembly notes before you order the kit. It is rare that someone has trouble assembling one.
4) Since most of the components are pretty small, I'd recommend a lower wattage iron with the smallest tip you can find. I use the Weller WCL100 soldering station set to about half heat with a ST6 or ST8 tip. If you do use the 40W iron, be careful not to overheat anything.
5) I still use 60/40 "44" solder in the .03 diameter. A smaller diameter might be a little easier to work with. I have not switched to lead free solder because the higher temperature places more stress on the components, and I am concerned about the growth of "tin whiskers" that are more of a problem with that solder.
The XPPF filter is available very inexpensively on eBay. I have some in stock that can be added to an XTB order for about half the "X10" price. (Mine come from the two major eBay sellers, who I highly recommend.)
FYI to all: With a little break away from the soldering iron, I'm back working on the firmware update again that will add the ability to repeat the "doublet" extended codes produced by the CM15A. While that should have been a simple mod, the unintended side effects are still being debugged.
Also FYI: The last letter in my name is a "p". However, that has been a common problem over the years, especially in this business.
Thanks again to all, and have a great new year!
Jeff