Generally I find the list price for scuba gear in Thailand about 30-50% more expensive than leisurepro or diveinn.
Unless you are a regular customer, you won't find much bargaining power like you do in Singapore or Hong Kong.
However many stores do offer 20-30% discount at certain time of the year which then make the prices more comparable to Leisurepro. For a first timer though, I doubt you are going to get better price than that of Leisurepro.
However since you specifically mention Suunto, beware that when you buy from Leisurepro, the manufacturer warranty will be invalid but Leisurepro does offer its own warranty. Something to do with price controlling in North America ie you can't be an authorized Suunto dealer if you offer more than certain percentage of discount from the manufacturer's suggested retail price. Thailand's price would probably be much more comparable to an authorized dealer in North America. Don['t forget that since you are a tourist, you should be entitle to a 7% Value Added Tax refund when you leave the country. Check with the stgore before you buy anything to make sure that they will issue you a VAT refund form (not every store will do this so don't assume that you will automatically get it). Most prices in Thailand already include VAT unless otherwise specified.