- It has some problems switching gears effectively;
- It consumes a lot of fuel;
- The ignition sound is harsh and loud;
- Idling has gone a bit up;

l have no idea, my best advice is to ask your question at an impreza forum. Here is one that l know is very popular ;

http://imprezasport.net/forums/index.php?act=home

As said best place to try is the Subaru forums.

But when was ur last service,If its due then it might be a simple as that.When l got my R33 it hadnt been serviced for a good while & performance was great but a little slow.
After a good service just decent oils, plugs, filters inc fuel filter,the idling dropped & perfomance was a lot smoother & a noticable difference.
Is ur an auto even changing the engine oil & plugs will ,make a difference in the smooth change.If u can also check the gear box oil maybe a new oil & filter service is due there also.
As said as iam sure u know dont use cheap oils
http://opieoils.co.uk/
f u have ur own place fair enough but not sales manager for them lol, l was told about these guys & go there all the time .worth a mail or phone to check prices etc.

firstly all imprezas use alot of fuel even non turbo models use alot.could do with an oil change or top up in gearbox that may solve your gearbox problems.sounds like engine could do with a service.and some new plugs.could be idling fast as you may need a new airflow meter.that would explain the fast idling.impreza owner of 12 years

check your oxygen sensor. A bad O2 sensor would cause the increased fuel usage, and poor idle.

Change the transmission oil, using the factory fill gear oil. the subaru part number is K0323AA093. It is hard to get a hold of, but well worth it.

as you are an impreza driver i'm sure you will be aware of the importance of regular services so l will not mention oil ect.

the forums l use,
http://sidc.co.uk/
http://scoobycity.co.uk/
http://scoobynet.com/

sidc is a really good 1 and if u post this question here it will get answered very quickly

When is the last time you got the oil replaced?

If you have a lot of money, and love the car, try having the engine rebuilt. Now that l think about it, having the engine rebuilt would be a waste of money. Still consider it though.