Z-Wave replacement of Insteon ToggleLincs

So, I own a large home (over 4000 sq. ft.) and currently have, among other devices on my Indigo network, 4 Insteon KeypadLincs controlling multiple lights each, and 15 Insteon ToggleLinc Dimmers. The ToggleLincs have been progressively failing (some are more than 10 years old), and apparently SmartHome is no longer manufacturing ToggleLincs. By failing I mean the switches take multiple (sometimes 5-10) tries to complete a command or answer a status request, and in some cases they just stop answering any commands from Indigo. Before anyone suggests switching to paddle switches, I'll mention that the house had over 70 toggle switches when built in 1999 (of which the above 15-20 are now a mix of KeypadLincs and ToggleLincs), so there's no way I'm switching out all our switches, and there's no way my wife would allow a mix of switch types (and I agree).

Before I decide to add a Z-Wave controller and start replacing the ToggleLincs, I want to know how close the Z-Wave toggle switches need to be to my Mac / Z-Wave main controller to be reliable, and how closely spaced they should be? I know the specification is 30 ft. indoors, but what is the real world experience that folks here have seen. Note I have several Insteon RF repeaters on different legs of the power (two 200 A main breaker boxes, so 4 arms of 110V power through the house), and I have still always had some reliability issues with commands to Insteon devices throughout the house.

I find Insteon to be less reliable then z-wave especially in large installations. (I could never get good reliability with Insteon, and I tried hard) I'm in the process of replacing Insteon devices with z-wave in my house (over 4k sqft) with devices also being in the back yard. I have replaced my sauna switch with z-wave at around 30ft away from the house. This is the only z-wave device that I had some issues with. I have an old house with stone walls so not very RF friendly. I managed to get perfect reliability by placing a z-wave repeater right outside of the house. I haven't had any issues since then. I'm not replacing all my insteon devices all at once because of the cost. Also at this point with a lot of Insteon devices replaced (especially wireless and the older generation) things run very well, so I'm not really in great hurry at this point.

Can't answer the specific question about distances but would just like to point out a simple solution.

First, understand that the switches themselves act as relays. So a switch say 20' away from the controller extends the range further in that direction.
Second, a powered z-wave device will also function as a relay. That means you could simply plug in something like a plug-in module between your controller and your furthest devices. They are unobtrusive and easy to install.

Anyway, perhaps by starting with the switches closest to the controller you'll get a good feel for how the system works overall.