Coils will not increase your power so don't waste your money on expensive coils.Maker's that claim all sorts of things are just using hyperbole.There are far too many parameters that affect performance and these bikes/engines were not deigned as sports engines.Some have been modified for racing but at great time and expense.
Things you can do.
Lose weight.I know it sounds simple but if you are carrying extra mass you are reducing the efficiency of the engine.I have deliberately,as I do,lost weight for the season from 13.5 st = 189 lb to 12 st = 168 lb
Take the centre stand off.It weighs loads.You will then need a jack or a motorcycle lift or similar for some bike work and make sure the side stand is in 100% good state.
As this is a CDI ignition bike you might get a few more Bhp by modifying the Advance curve but to do that effectively you need one of the Ignitech ignition units,
http://www.ignitech.cz/english/aindex.htmbut there are better reasons to fit one if you have money to spare and of course get it wrong and you can ruin the engine plus they are 30 year+ old metal.
Head porting.Some people claim they have got better fuel mileage or power from polishing both inlet and exhaust ports.Of course there are counter claims.My take on it is that all racing machines and drag bikes(One of my best pals is a Drag bike racer) use port polishing so there must be something in it.
Pod filters and re-jetting.Again this may increase power at certain rev-bands but of course will lose some at others.
Because these are shaft drive machines it's very hard to try different output ratios at the rear wheel.The smaller 16" wheels on the GLs and Customs I assume must give better acceleration than the 18" wheels but at the cost of economy and overall performance.
Electric Fan conversion.
This has made my CX zippier,
viewtopic.php?f=7&t=209and of course I don't need that bloody silly mechanical fan and fan guard
That's all I can think of for now.