Its all in the title.

An air inline filter kit, a straight through exhaust, and possibly a chip, but l think you can only do that with the HDi (common rail) engines. You could also upgrade the turbo if you have an unlimited budget.

buy a tuning chip on ebay to increase power by up to 40 bhp

if u dont have that much money u could buy cold air intake, exsaust system, headers, performence chip

If it is the HD l engine. You can have the ECU Re-mapped, this will give you around 115-120 BHP.

If it is the old Turbo Diesel engine. Depending on the fuel pump, you can alter the fuel settings and increase the turbo pressure. This will give you around 105-110 BHP.

Visit pug306.net for more information :)

The 306 diesel gearbox can be replaced by one of several from old 205 models. (the M5 gearboxes). This will give u a lower gear ratio in the lower gears.you can squeel the tires from a starting line. POWER!!!

Then the final 5th gear could also be changed for lower rpm is at high speeds, giving u better fuel useage & less engine noise on the motorway.

Hear is a list of all M5 transmissions by Peugeot & their gear ratios. Find yours, then discuss a replacement with a knowledgeable mechanic..
http://suzukituning.com/PSA/MA5%20Gearbox/ma5gearratio.htm

Here is the procedure to change the 5th gear inside the gearbox:
http://suzukituning.com/PSA/MA5%20Gearbox/5th%20gear%20change.htm