Does anyone know whether doubling the section from Ladybank to Perth would much difference to journey times?
Yes, but only primarily between Perth and Newburgh, due to the tight curves between Newburgh and Collessie - although the latter might be able to see a minor increase.
Bridge of Earn to Newburgh could easily support 100mph operation if the line was lifted and doubled up. One of the barriers affecting journey times between Perth and Edinburgh is the rolling stock at ScotRail's disposal. You've got 90mph limited 158s and 170s which have crap acceleration, running on 60mph max lines for a chunk of the journey.
With the £1.5b price tag, I guess it begs the question as to what could be done with the existing route before turning to a new route as the last resort. For example (proposals aside), if you look at the rolling stock, in theory, Perth to Edinburgh would be a perfect route for the Voyagers in every way possible. Along with a new cord from Halbeath to Inverkeithing and doubling Ladybank to Perth, it goes to show that there is still room for improvement on the existing line in order to cut journey times to Perth down significantly, which can save you the better part of £1.5b.
It's whether or not the Scottish Government deems a new route to be more cost effective and beneficial as a whole, than investing in the current infrastructure further - and in this case, a new route via Kinross would probably be the best solution in the long term, albeit mighty expensive.
Following the M90 seems like it would be a lot of work between junctions 3 and 4 - might be easier to go via the old line from Cowdenbeath passing east of Kelty and between the M90 and Loch Leven. But then what do you do at Kinross - squeeze in next to the M90 near the old alignment? Has there ever been a map of this proposed line produced anywhere?
I only really meant following the principle places the M90 passes, rather than following that motorway turn for turn.
Kinross would require a tunnel of a good mile or two long, due to the limitations of trying to squeeze through the old site as a result of the M90.