hi Tac.
I doubt they are different scales.
I believe the boiler mouldings are the same.  The current model still has the internal mouldings to mount the original motor block.
Have you actually sat them side by side to check for size difference and done a thorough measurement comparison?

Terry.
The GST free threshold for mail items into Australia is A$1,000 so the assessment from £95.00 to A$500 +, should have made no difference to the cost of importing the parcel.
Customs Duty may be payable but should never be more than 71/2 %.  So, if ever sending model RR related items via the mail always quote the TC #9503.00.00, then no import duty is payable.

Both sound systems require a two wire connection for the chuff to work.
One of the wires from the sound is hot (logic high) and is taken to ground to complete the circuit.
Therefore at some point the Phoenix must also be connected to the loco ground (-).

Use a multi meter to determine which is the ground connection of the Dallee sound and connect it to the loco ground (-).
The other Dallee chuff wire goes to the C 19 chuff sensor.

When the Bachmann K-27 came out a good engineer friend of mine inspected it minutely.
His opinion was the K-27 chassis was exceptionally well thought out and built and quite robust enough for an electric model.  Unfortunately it was not suitable for live steam as it was.
If the engineering quality of the drive train was improved so as to be suitable live steam use, then, perhaps yes it would be.

To the best of my knowledge Hobby King (HK) do not sell Tenergy batteries.  Patrick had it right the first time.  The HK brand is Turnigy.
The Phoenix P8 instructions do not list a minimum voltage in the tech specs on page # 14.  Only a maximum voltage.  30 volts.
The only reference to a minimum voltage is during initial checkout on page # 6 where it states "When the power rises above 9 volts you should hear the sound"

Can't help you with a track and battery conversion.  Once converted to battery power very few users ever stick with the track power option.
Here is one way to do battery only power.
http://www.rcs-rc.com/app/webroot/PDF/Acessories/Installation-kits/BIK-ANNIE.pdf

If it is an early Shay version there are internal motor block connections from the track to the motor that need to be rewired.  There is no quick and easy solution short of getting into the truck to do this.
You will need to re-route all the track pick ups to one side of a DPDT switch.  The battery powered ESC output wires go to the other side of the switch.  The centre terminals of the switch go back to the loco wiring.
It is much much simpler to forget about about track power, disconnect the track pick ups by removing the ball bearing pick ups and wire the ESC output to what were the wires from the track pick ups.  That can be done without too much trouble.

That is the beauty of the hobby.
Model Railroading opens up the ability to express so much creativity.
I love the oddities.  Usually adapted from the equipment originally sold in other scales.
Long live the kitbasher.

Bill.
I think you will find Gn15 is 1:22.5 scale running on H0 track to represent 15" gauge estate railways.

For regular DC track voltage and not DCC, the answers are No and Yes.
You might even be able to sell the fitted Tsunami decoder.

Hello Dave.
You will have to invert the chuff signal to make it work with a P8, PB9 or PB11 using on board battery power and Radio Control.  Not sure how this will work with regular track power.

No need to build a special pcb.  You can add the two components direct to the stock Bachmann socket pcb.
All this takes is a small open collector transistor (I use a BC337) and a .1 mfd filtering capacitor.
This shows how to wire it up.

http://www.rcs-rc.com/app/webroot/store/PDF/Instructions/Diagrams/PnP-BKv3.pdf

I rarely buy anything for myself but I would buy anything modelled from the SP narrow gauge.
Great dismal, great steamers  and great rolling stock.
Much of it still preserved in the Owens Valley.
Different from the Colorado narrow gauge, but a perfect example of narrow gauge equipment.
Bring it on.

The P8 is also designed for track powered DCC.
It is not designed for variable voltage DC track power. 
The PB11 will run on regular variable DC track voltage.

Once  upon a time all am stick radio TX's would work with any brand of am RX.  Not any more now 2.4 Ghz is here.  None of the different brands of 2.4 GHz R/C are compatible with one another.  In fact commonality of stick radios R/C is further away than it ever was.
I doubt very much a new, different from what there is already control and sound system, would ever be accepted by the market place.
It would be like MTH DCS being adopted instead of DCC and made compulsory.
It just isn't going to happen.

Bill,
After the page # 5 full page ad in May - June issue of GR, Bachmann ran the same full page ad for at least the next three issues.
I bought my THE BIG HAULER battery R/C set from a hobby shop in LA during a visit to the USA for the 1988 Garden Railway and Narrow Gauge Conventions. Unfortunately I cannot remember off hand the name of the hobby shop.